diff --git a/netwerk/cache/nsDiskCacheMap.cpp b/netwerk/cache/nsDiskCacheMap.cpp
index b8741e8ef4e..c564e238b92 100644
--- a/netwerk/cache/nsDiskCacheMap.cpp
+++ b/netwerk/cache/nsDiskCacheMap.cpp
@@ -234,7 +234,10 @@ nsDiskCacheMap::FlushHeader()
     if (sizeof(nsDiskCacheHeader) != bytesWritten) {
         return NS_ERROR_UNEXPECTED;
     }
-    
+
+    PRStatus err = PR_Sync(mMapFD);
+    if (err != PR_SUCCESS) return NS_ERROR_UNEXPECTED;
+
     return NS_OK;
 }