diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index c6924e2cb0a..872aad6b046 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -4526,7 +4526,7 @@ namespace ts {
          * type itself. Note that the apparent type of a union type is the union type itself.
          */
         function getApparentType(type: Type): Type {
-            let t = type.flags & TypeFlags.TypeParameter ? getApparentTypeOfTypeParameter(<TypeParameter>type) : type;
+            const t = type.flags & TypeFlags.TypeParameter ? getApparentTypeOfTypeParameter(<TypeParameter>type) : type;
             return t.flags & TypeFlags.StringLike ? globalStringType :
                 t.flags & TypeFlags.NumberLike ? globalNumberType :
                 t.flags & TypeFlags.BooleanLike ? globalBooleanType :