diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 706ed1c65a3..c4a60cd3355 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -3731,7 +3731,7 @@ namespace ts {
             if (node.initializer) {
                 const signatureDeclaration = <SignatureDeclaration>node.parent;
                 const signature = getSignatureFromDeclaration(signatureDeclaration);
-                const parameterIndex = signatureDeclaration.parameters.indexOf(node);
+                const parameterIndex = ts.indexOf(signatureDeclaration.parameters, node);
                 Debug.assert(parameterIndex >= 0);
                 return parameterIndex >= signature.minArgumentCount;
             }