diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 1ca1cd9cf74..450fcfe1da4 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -18411,8 +18411,7 @@ namespace ts {
                     if (produceDiagnostics) {
                         const t = getTypeFromTypeNode(typeRefNode);
                         if (t !== unknownType) {
-                            const declaredType = getObjectFlags(t) & ObjectFlags.Reference ? (<TypeReference>t).target : t;
-                            if (getObjectFlags(declaredType) & ObjectFlags.ClassOrInterface) {
+                            if (isValidBaseType(t)) {
                                 checkTypeAssignableTo(typeWithThis, getTypeWithThisArgument(t, type.thisType), node.name || node, Diagnostics.Class_0_incorrectly_implements_interface_1);
                             }
                             else {