diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 40db300d310..3dc9ee68d14 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -6362,9 +6362,10 @@ namespace ts {
 
             function narrowTypeByInstanceof(type: Type, expr: BinaryExpression, assumeTrue: boolean): Type {
                 // Check that type is not any, assumed result is true, and we have variable symbol on the left
-                if (isTypeAny(type) || !assumeTrue || expr.left.kind !== SyntaxKind.Identifier || getResolvedSymbol(<Identifier>expr.left) !== symbol) {
+                if (isTypeAny(type) || expr.left.kind !== SyntaxKind.Identifier || getResolvedSymbol(<Identifier>expr.left) !== symbol) {
                     return type;
                 }
+
                 // Check that right operand is a function type with a prototype property
                 let rightType = checkExpression(expr.right);
                 if (!isTypeSubtypeOf(rightType, globalFunctionType)) {
@@ -6396,6 +6397,13 @@ namespace ts {
                 }
 
                 if (targetType) {
+                    if (!assumeTrue) {
+                        if (type.flags & TypeFlags.Union) {
+                            return getUnionType(filter((<UnionType>type).types, t => !isTypeSubtypeOf(t, targetType)));
+                        }
+                        return type;
+                    }
+
                     return getNarrowedType(type, targetType);
                 }