diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 64e771c7c9f..aa4796c2c58 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -8971,7 +8971,7 @@ namespace ts {
             return undefined;
         }
 
-        function getConstraintDeclaration(type: TypeParameter) {
+        function getConstraintDeclaration(type: TypeParameter): TypeNode | undefined {
             return mapDefined(filter(type.symbol && type.symbol.declarations, isTypeParameterDeclaration), getEffectiveConstraintOfTypeParameter)[0];
         }