diff --git a/src/compiler/factory.ts b/src/compiler/factory.ts
index 7c98dba8494..ed04a6dc6f0 100644
--- a/src/compiler/factory.ts
+++ b/src/compiler/factory.ts
@@ -4340,7 +4340,7 @@ namespace ts {
         }
     }
 
-    function getLeftmostExpression(node: Expression, stopAtCallExpressions: boolean | undefined) {
+    function getLeftmostExpression(node: Expression, stopAtCallExpressions: boolean) {
         while (true) {
             switch (node.kind) {
                 case SyntaxKind.PostfixUnaryExpression: