diff --git a/lang/core/iter/filter.go b/lang/core/iter/filter.go
index a92589a7..5123e06f 100644
--- a/lang/core/iter/filter.go
+++ b/lang/core/iter/filter.go
@@ -449,7 +449,9 @@ func (obj *FilterFunc) Call(ctx context.Context, args []types.Value) (types.Valu
 
 	// Every time the FuncValue or the length of the list changes, recreate
 	// the subgraph, by calling the FuncValue N times on N nodes, each of
-	// which extracts one of the N values in the list.
+	// which extracts one of the N values in the list. If the contents of
+	// the list change (BUT NOT THE LENGTH) then it's okay to use the
+	// existing graph, because the shape is the same!
 
 	n := len(newInputList.List())
 
@@ -459,10 +461,6 @@ func (obj *FilterFunc) Call(ctx context.Context, args []types.Value) (types.Valu
 	}
 	obj.lastInputListLength = n
 
-	if b && !c { // different length list
-		return types.NewNil(), nil // dummy value
-	}
-
 	// If we have a new function or the length of the input list has
 	// changed, then we need to replace the subgraph with a new one that
 	// uses the new function the correct number of times.
diff --git a/lang/core/iter/map.go b/lang/core/iter/map.go
index e1fc6b78..40fa5050 100644
--- a/lang/core/iter/map.go
+++ b/lang/core/iter/map.go
@@ -393,7 +393,9 @@ func (obj *MapFunc) Call(ctx context.Context, args []types.Value) (types.Value,
 
 	// Every time the FuncValue or the length of the list changes, recreate
 	// the subgraph, by calling the FuncValue N times on N nodes, each of
-	// which extracts one of the N values in the list.
+	// which extracts one of the N values in the list. If the contents of
+	// the list change (BUT NOT THE LENGTH) then it's okay to use the
+	// existing graph, because the shape is the same!
 
 	n := len(newInputList.List())
 
@@ -403,10 +405,6 @@ func (obj *MapFunc) Call(ctx context.Context, args []types.Value) (types.Value,
 	}
 	obj.lastInputListLength = n
 
-	if b && !c { // different length list
-		return types.NewNil(), nil // dummy value
-	}
-
 	// If we have a new function or the length of the input list has
 	// changed, then we need to replace the subgraph with a new one that
 	// uses the new function the correct number of times.