diff --git a/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/ITypeCalculator.java b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/ITypeCalculator.java
index e1f0a79f7..4809da4be 100644
--- a/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/ITypeCalculator.java
+++ b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/ITypeCalculator.java
@@ -1,7 +1,9 @@
 package org.eclipse.epsilon.eol.execute.operations;
 
+import java.util.List;
+
 import org.eclipse.epsilon.eol.staticanalyser.types.EolType;
 
 public interface ITypeCalculator {
-public EolType calculateType(EolType contextType, EolType iteratorType, EolType expressionType);
+public EolType calculateType(EolType contextType, EolType iteratorType, List<EolType> expressionTypes);
 }
diff --git a/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/AggregateOperation.java b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/AggregateOperation.java
index 42e9ab95a..d9961eca1 100644
--- a/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/AggregateOperation.java
+++ b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/AggregateOperation.java
@@ -20,8 +20,10 @@
 import org.eclipse.epsilon.eol.execute.context.FrameType;
 import org.eclipse.epsilon.eol.execute.context.IEolContext;
 import org.eclipse.epsilon.eol.execute.context.Variable;
+import org.eclipse.epsilon.eol.execute.operations.TypeCalculator;
 import org.eclipse.epsilon.eol.types.EolMap;
 
+@TypeCalculator(klass = AggregateTypeCalculator.class)
 public class AggregateOperation extends FirstOrderOperation {
 	
 	@Override
diff --git a/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/AggregateTypeCalculator.java b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/AggregateTypeCalculator.java
new file mode 100644
index 000000000..8c732f109
--- /dev/null
+++ b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/AggregateTypeCalculator.java
@@ -0,0 +1,16 @@
+package org.eclipse.epsilon.eol.execute.operations.declarative;
+
+import java.util.List;
+
+import org.eclipse.epsilon.eol.execute.operations.ITypeCalculator;
+import org.eclipse.epsilon.eol.staticanalyser.types.EolMapType;
+import org.eclipse.epsilon.eol.staticanalyser.types.EolType;
+
+public class AggregateTypeCalculator implements ITypeCalculator {
+
+	@Override
+	public EolType calculateType(EolType contextType, EolType iteratorType, List<EolType> expressionTypes) {
+		return new EolMapType(expressionTypes.get(0), expressionTypes.get(1));
+	}
+
+}
diff --git a/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/BooleanTypeCalculator.java b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/BooleanTypeCalculator.java
index dab240edc..ee41c1ff7 100644
--- a/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/BooleanTypeCalculator.java
+++ b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/BooleanTypeCalculator.java
@@ -1,5 +1,7 @@
 package org.eclipse.epsilon.eol.execute.operations.declarative;
 
+import java.util.List;
+
 import org.eclipse.epsilon.eol.execute.operations.ITypeCalculator;
 import org.eclipse.epsilon.eol.staticanalyser.types.EolPrimitiveType;
 import org.eclipse.epsilon.eol.staticanalyser.types.EolType;
@@ -7,7 +9,7 @@
 public class BooleanTypeCalculator implements ITypeCalculator {
 
 	@Override
-	public EolType calculateType(EolType contextType, EolType iteratorType, EolType expressionType) {
+	public EolType calculateType(EolType contextType, EolType iteratorType, List<EolType> expressionTypes) {
 		return EolPrimitiveType.Boolean;
 	}
 
diff --git a/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/CollectTypeCalculator.java b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/CollectTypeCalculator.java
index 4cce66d60..90a42b359 100644
--- a/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/CollectTypeCalculator.java
+++ b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/CollectTypeCalculator.java
@@ -1,5 +1,7 @@
 package org.eclipse.epsilon.eol.execute.operations.declarative;
 
+import java.util.List;
+
 import org.eclipse.epsilon.eol.execute.operations.ITypeCalculator;
 import org.eclipse.epsilon.eol.staticanalyser.types.EolCollectionType;
 import org.eclipse.epsilon.eol.staticanalyser.types.EolType;
@@ -7,7 +9,7 @@
 public class CollectTypeCalculator implements ITypeCalculator {
 
 	@Override
-	public EolType calculateType(EolType contextType, EolType iteratorType, EolType expressionType) {
+	public EolType calculateType(EolType contextType, EolType iteratorType, List<EolType> expressionTypes) {
 		String collectionName = ((EolCollectionType)contextType).getName();
 		String newCollectionName = null;
 		if (collectionName.equals("Bag") || collectionName.equals("Set")) {
@@ -19,7 +21,7 @@ else if (collectionName.equals("Sequence") || collectionName.equals("OrderedSet"
 		else {
 			throw new RuntimeException("Unknown collection name");
 		}
-		return new EolCollectionType(newCollectionName, expressionType);
+		return new EolCollectionType(newCollectionName, expressionTypes.get(0));
 	}
 
 }
diff --git a/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/IntegerTypeCalculator.java b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/IntegerTypeCalculator.java
index b71677a66..091dc62ac 100644
--- a/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/IntegerTypeCalculator.java
+++ b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/IntegerTypeCalculator.java
@@ -1,5 +1,7 @@
 package org.eclipse.epsilon.eol.execute.operations.declarative;
 
+import java.util.List;
+
 import org.eclipse.epsilon.eol.execute.operations.ITypeCalculator;
 import org.eclipse.epsilon.eol.staticanalyser.types.EolPrimitiveType;
 import org.eclipse.epsilon.eol.staticanalyser.types.EolType;
@@ -7,7 +9,7 @@
 public class IntegerTypeCalculator implements ITypeCalculator {
 
 	@Override
-	public EolType calculateType(EolType contextType, EolType iteratorType, EolType expressionType) {
+	public EolType calculateType(EolType contextType, EolType iteratorType, List<EolType> expressionTypes) {
 		return EolPrimitiveType.Integer;
 	}
 
diff --git a/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/MapByTypeCalculator.java b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/MapByTypeCalculator.java
index 67148a715..4123da5c6 100644
--- a/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/MapByTypeCalculator.java
+++ b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/MapByTypeCalculator.java
@@ -1,5 +1,7 @@
 package org.eclipse.epsilon.eol.execute.operations.declarative;
 
+import java.util.List;
+
 import org.eclipse.epsilon.eol.execute.operations.ITypeCalculator;
 import org.eclipse.epsilon.eol.staticanalyser.types.EolCollectionType;
 import org.eclipse.epsilon.eol.staticanalyser.types.EolMapType;
@@ -8,8 +10,8 @@
 public class MapByTypeCalculator implements ITypeCalculator {
 
 	@Override
-	public EolType calculateType(EolType contextType, EolType iteratorType, EolType expressionType) {
-		return new EolMapType(expressionType, new EolCollectionType("Sequence", iteratorType));
+	public EolType calculateType(EolType contextType, EolType iteratorType, List<EolType> expressionTypes) {
+		return new EolMapType(expressionTypes.get(0), new EolCollectionType("Sequence", iteratorType));
 	}
 
 }
diff --git a/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/SelectOneTypeCalculator.java b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/SelectOneTypeCalculator.java
index abf92a5d5..532d5d560 100644
--- a/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/SelectOneTypeCalculator.java
+++ b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/SelectOneTypeCalculator.java
@@ -1,12 +1,14 @@
 package org.eclipse.epsilon.eol.execute.operations.declarative;
 
+import java.util.List;
+
 import org.eclipse.epsilon.eol.execute.operations.ITypeCalculator;
 import org.eclipse.epsilon.eol.staticanalyser.types.EolType;
 
 public class SelectOneTypeCalculator implements ITypeCalculator {
 
 	@Override
-	public EolType calculateType(EolType contextType, EolType iteratorType, EolType expressionType) {
+	public EolType calculateType(EolType contextType, EolType iteratorType, List<EolType> expressionType) {
 		return iteratorType;
 	}
 
diff --git a/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/SelectTypeCalculator.java b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/SelectTypeCalculator.java
index 58b456e27..a238ef687 100644
--- a/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/SelectTypeCalculator.java
+++ b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/SelectTypeCalculator.java
@@ -1,5 +1,7 @@
 package org.eclipse.epsilon.eol.execute.operations.declarative;
 
+import java.util.List;
+
 import org.eclipse.epsilon.eol.execute.operations.ITypeCalculator;
 import org.eclipse.epsilon.eol.staticanalyser.types.EolCollectionType;
 import org.eclipse.epsilon.eol.staticanalyser.types.EolType;
@@ -7,7 +9,7 @@
 public class SelectTypeCalculator implements ITypeCalculator {
 
 	@Override
-	public EolType calculateType(EolType contextType, EolType iteratorType, EolType expressionType) {
+	public EolType calculateType(EolType contextType, EolType iteratorType, List<EolType> expressionTypes) {
 		String collectionName = ((EolCollectionType)contextType).getName();
 		return new EolCollectionType(collectionName, iteratorType);
 		
diff --git a/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/SortByTypeCalculator.java b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/SortByTypeCalculator.java
index e91a82794..670272f5e 100644
--- a/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/SortByTypeCalculator.java
+++ b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/operations/declarative/SortByTypeCalculator.java
@@ -1,5 +1,7 @@
 package org.eclipse.epsilon.eol.execute.operations.declarative;
 
+import java.util.List;
+
 import org.eclipse.epsilon.eol.execute.operations.ITypeCalculator;
 import org.eclipse.epsilon.eol.staticanalyser.types.EolCollectionType;
 import org.eclipse.epsilon.eol.staticanalyser.types.EolType;
@@ -7,8 +9,7 @@
 public class SortByTypeCalculator implements ITypeCalculator {
 
 	@Override
-	public EolType calculateType(EolType contextType, EolType iteratorType, EolType expressionType) {
+	public EolType calculateType(EolType contextType, EolType iteratorType, List<EolType> expressionTypes) {
 		return new EolCollectionType("Sequence", iteratorType);
 	}
-
 }
diff --git a/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/staticanalyser/EolStaticAnalyser.java b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/staticanalyser/EolStaticAnalyser.java
index c1b1d4538..70f4b9d08 100644
--- a/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/staticanalyser/EolStaticAnalyser.java
+++ b/plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/staticanalyser/EolStaticAnalyser.java
@@ -291,13 +291,16 @@ public void visit(FirstOrderOperationCallExpression firstOrderOperationCallExpre
 
 		context.getFrameStack().enterLocal(FrameType.UNPROTECTED, firstOrderOperationCallExpression,
 				new Variable(iterator.getName(), iteratorType));
-		Expression expression = firstOrderOperationCallExpression.getExpressions().get(0);
-		expression.accept(this);
-		EolType expressionType = getResolvedType(expression);
+		List<Expression> expressions = firstOrderOperationCallExpression.getExpressions();
+		for (Expression expression: expressions) {
+			expression.accept(this);
+		}
+		
+		List<EolType> expressionTypes = expressions.stream().map(e -> getResolvedType(e)).collect(Collectors.toList());
 		context.getFrameStack().leaveLocal(firstOrderOperationCallExpression);
 
 		try {
-			EolType returnType = tc.klass().newInstance().calculateType(contextType, iteratorType, expressionType);
+			EolType returnType = tc.klass().newInstance().calculateType(contextType, iteratorType, expressionTypes);
 			setResolvedType(firstOrderOperationCallExpression, returnType);
 		} catch (Exception e) {
 			setResolvedType(firstOrderOperationCallExpression, EolAnyType.Instance);
diff --git a/tests/org.eclipse.epsilon.eol.staticanalyser.tests/src/org/eclipse/epsilon/eol/staticanalyser/tests/scripts/aggregate.eol b/tests/org.eclipse.epsilon.eol.staticanalyser.tests/src/org/eclipse/epsilon/eol/staticanalyser/tests/scripts/aggregate.eol
new file mode 100644
index 000000000..712858d87
--- /dev/null
+++ b/tests/org.eclipse.epsilon.eol.staticanalyser.tests/src/org/eclipse/epsilon/eol/staticanalyser/tests/scripts/aggregate.eol
@@ -0,0 +1,4 @@
+/*Map<Integer, Integer>*/Bag{0..9}.aggregate(i:Integer|i,i);
+/*Map<Integer, Integer>*/Sequence{0..9}.aggregate(i:Integer|i,i);
+/*Map<Integer, Integer>*/Set{0..9}.aggregate(i:Integer|i,i);
+/*Map<Integer, Integer>*/OrderedSet{0..9}.aggregate(i:Integer|i,i);
\ No newline at end of file