diff --git a/clippy_lints/src/unnecessary_reserve.rs b/clippy_lints/src/unnecessary_reserve.rs
index 015e58c1f07f..9caf0250dea7 100644
--- a/clippy_lints/src/unnecessary_reserve.rs
+++ b/clippy_lints/src/unnecessary_reserve.rs
@@ -52,11 +52,11 @@ impl<'tcx> LateLintPass<'tcx> for UnnecessaryReserve {
             return;
         }
 
-        if let ExprKind::MethodCall(PathSegment { ident: method, .. }, struct_calling_on, _, _) = expr.kind
+        if let ExprKind::MethodCall(PathSegment { ident: method, .. }, struct_calling_on, args_a, _) = expr.kind
                 && method.name.as_str() == "reserve"
                 && acceptable_type(cx, struct_calling_on)
                 && let Some(block) = get_enclosing_block(cx, expr.hir_id)
-                && let Some(next_stmt_span) = check_extend_method(cx, block, struct_calling_on)
+                && let Some(next_stmt_span) = check_extend_method(cx, block, struct_calling_on, &args_a[0])
                 && !next_stmt_span.from_expansion()
             {
                 span_lint_and_then(
@@ -95,13 +95,17 @@ fn check_extend_method(
     cx: &LateContext<'_>,
     block: &Block<'_>,
     struct_expr: &rustc_hir::Expr<'_>,
+    args_a: &rustc_hir::Expr<'_>,
 ) -> Option<rustc_span::Span> {
+    let args_a_kind = &args_a.kind;
     let mut read_found = false;
     let mut spanless_eq = SpanlessEq::new(cx);
 
     let _: Option<!> = for_each_expr(block, |expr| {
         if let ExprKind::MethodCall(_, struct_calling_on, _,_) = expr.kind
             && let Some(expr_def_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id)
+            && let ExprKind::MethodCall(PathSegment { ident: method_call_a, .. },..) = args_a_kind
+            && method_call_a.name.as_str() == "len"
             && match_def_path(cx, expr_def_id, &paths::ITER_EXTEND)
             && acceptable_type(cx, struct_calling_on)
             // Check that both expr are equal
diff --git a/tests/ui/unnecessary_reserve.stderr b/tests/ui/unnecessary_reserve.stderr
index 75308423d020..23cb227924cc 100644
--- a/tests/ui/unnecessary_reserve.stderr
+++ b/tests/ui/unnecessary_reserve.stderr
@@ -1,21 +1,3 @@
-error: unnecessary call to `reserve`
-  --> $DIR/unnecessary_reserve.rs:14:18
-   |
-LL |   fn vec_reserve() {
-   |  __________________^
-LL | |     let mut vec: Vec<usize> = vec![];
-LL | |     let array: &[usize] = &[1, 2];
-LL | |
-LL | |     // do not lint
-LL | |     vec.reserve(1);
-   | |     -------------- help: remove this line
-...  |
-LL | |     vec.extend([1])
-LL | | }
-   | |_^
-   |
-   = note: `-D clippy::unnecessary-reserve` implied by `-D warnings`
-
 error: unnecessary call to `reserve`
   --> $DIR/unnecessary_reserve.rs:14:18
    |
@@ -31,6 +13,8 @@ LL | |     vec.reserve(array.len());
 LL | |     vec.extend([1])
 LL | | }
    | |_^
+   |
+   = note: `-D clippy::unnecessary-reserve` implied by `-D warnings`
 
 error: unnecessary call to `reserve`
   --> $DIR/unnecessary_reserve.rs:27:5
@@ -58,22 +42,6 @@ LL | |     vec.extend([1])
 LL | | }
    | |_^
 
-error: unnecessary call to `reserve`
-  --> $DIR/unnecessary_reserve.rs:43:24
-   |
-LL |   fn vec_deque_reserve() {
-   |  ________________________^
-LL | |     let mut vec_deque: VecDeque<usize> = [1].into();
-LL | |     let array: &[usize] = &[1, 2];
-LL | |
-LL | |     // do not lint
-LL | |     vec_deque.reserve(1);
-   | |     -------------------- help: remove this line
-...  |
-LL | |     vec_deque.extend([1])
-LL | | }
-   | |_^
-
 error: unnecessary call to `reserve`
   --> $DIR/unnecessary_reserve.rs:43:24
    |
@@ -90,31 +58,5 @@ LL | |     vec_deque.extend([1])
 LL | | }
    | |_^
 
-error: unnecessary call to `reserve`
-  --> $DIR/unnecessary_reserve.rs:56:5
-   |
-LL | /     {
-LL | |         vec_deque.reserve(1);
-   | |         -------------------- help: remove this line
-LL | |         vec_deque.extend([1])
-LL | |     };
-   | |_____^
-
-error: unnecessary call to `reserve`
-  --> $DIR/unnecessary_reserve.rs:43:24
-   |
-LL |   fn vec_deque_reserve() {
-   |  ________________________^
-LL | |     let mut vec_deque: VecDeque<usize> = [1].into();
-LL | |     let array: &[usize] = &[1, 2];
-LL | |
-...  |
-LL | |     vec_deque.reserve(array.len() + 1);
-   | |     ---------------------------------- help: remove this line
-...  |
-LL | |     vec_deque.extend([1])
-LL | | }
-   | |_^
-
-error: aborting due to 8 previous errors
+error: aborting due to 4 previous errors