diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 0dacb20ece41c..957a3e3630aef 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -4379,6 +4379,8 @@ module ts {
                     }
                 }
 
+                Debug.assert(!!downfallType, "If there is no common supertype, each type should have a downfallType");
+
                 if (score > bestSupertypeScore) {
                     bestSupertype = types[i];
                     bestSupertypeDownfallType = downfallType;