From 4e56fc0d272e39706ea98bdff0ded79dc70ca1d6 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Mon, 22 Aug 2016 09:49:26 -0700
Subject: [PATCH] Properly guard for undefined in getTypeReferenceArity

---
 src/compiler/checker.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 9efaf72c03a..3abf271a57b 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -4979,7 +4979,7 @@ namespace ts {
         }
 
         function getTypeReferenceArity(type: TypeReference): number {
-            return type.target.typeParameters.length;
+            return type.target.typeParameters ? type.target.typeParameters.length : 0;
         }
 
         // Get type from reference to class or interface