diff --git a/lib/System/Win32/ThreadLocal.inc b/lib/System/Win32/ThreadLocal.inc
index 8ab37d9d75e..c8f7840b003 100644
--- a/lib/System/Win32/ThreadLocal.inc
+++ b/lib/System/Win32/ThreadLocal.inc
@@ -43,7 +43,7 @@ const void* ThreadLocalImpl::getInstance() {
 void ThreadLocalImpl::setInstance(const void* d){
   DWORD* tls = static_cast<DWORD*>(data);
   int errorcode = TlsSetValue(*tls, const_cast<void*>(d));
-  assert(errorcode == 0);
+  assert(errorcode != 0);
 }
 
 }