diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 6f5a3e67d0a..0a5525edaaf 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -6479,6 +6479,10 @@ namespace ts {
                     assumeTrue = !assumeTrue;
                 }
                 const typeInfo = primitiveTypeInfo[right.text];
+                // Don't narrow `undefined`
+                if (typeInfo && typeInfo.type === undefinedType) {
+                    return type;
+                }
                 // If the type to be narrowed is any and we're checking a primitive with assumeTrue=true, return the primitive
                 if (!!(type.flags & TypeFlags.Any) && typeInfo && assumeTrue) {
                     return typeInfo.type;