diff --git a/src/harness/harness.ts b/src/harness/harness.ts
index 88346557ba7..26a4a6f963d 100644
--- a/src/harness/harness.ts
+++ b/src/harness/harness.ts
@@ -2015,7 +2015,7 @@ namespace Harness {
 
     export function isDefaultLibraryFile(filePath: string): boolean {
         // We need to make sure that the filePath is prefixed with "lib." not just containing "lib." and end with ".d.ts"
-        const fileName = ts.getBaseFileName(filePath);
+        const fileName = ts.getBaseFileName(ts.normalizeSlashes(filePath));
         return ts.startsWith(fileName, "lib.") && ts.endsWith(fileName, ".d.ts");
     }