diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 27b0839720..b7779fc9da 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -5678,7 +5678,7 @@ namespace ts {
                         }
                     }
                     if (target.flags & TypeFlags.Union) {
-                        if (result = typeRelatedToSomeType(source, <UnionType>target, reportErrors)) {
+                        if (result = typeRelatedToSomeType(source, <UnionType>target, reportErrors && !(source.flags & TypeFlags.Primitive))) {
                             return result;
                         }
                     }