diff --git a/cylc/flow/xtriggers/workflow_state.py b/cylc/flow/xtriggers/workflow_state.py
index ca0a96a9aed..a55d5038886 100644
--- a/cylc/flow/xtriggers/workflow_state.py
+++ b/cylc/flow/xtriggers/workflow_state.py
@@ -114,9 +114,7 @@ def validate(args: Dict[str, Any]):
         raise WorkflowConfigError(
             "Full ID needed: workflow//cycle/task[:selector].")
 
-    try:
-        int(args["flow_num"])
-    except ValueError:
+    if not isinstance(args["flow_num"], int):
         raise WorkflowConfigError("flow_num must be an integer.")
 
 
diff --git a/tests/unit/xtriggers/test_workflow_state.py b/tests/unit/xtriggers/test_workflow_state.py
index cd537a11b7d..95a20f8b8c2 100644
--- a/tests/unit/xtriggers/test_workflow_state.py
+++ b/tests/unit/xtriggers/test_workflow_state.py
@@ -288,7 +288,7 @@ def test_validate_fail_bad_id(id_):
 
 def test_validate_fail_non_int_flow():
     """Validate returns ok  with valid args."""
-    with pytest.raises(ValueError, match='must be an integer'):
+    with pytest.raises(WorkflowConfigError, match='must be an integer'):
         validate({
             'workflow_task_id': 'foo//1/bar',
             'offset': 'PT1H',