--- a/WinWorkstation.st	Mon Oct 22 15:03:13 2018 +0200
+++ b/WinWorkstation.st	Mon Oct 22 15:05:08 2018 +0200
@@ -16681,8 +16681,11 @@
 %{  /* STACK: 100000 */ 
     if (__isExternalAddress(dataAddress)) {
 	    fileDialogData *pFdd = __externalAddressVal(dataAddress);
-        EnumWindows(enumWindowsToFindAndDestroyFileDialogProc, pFdd->fileDialogThreadId);
+		DWORD fileDialogThreadId = pFdd->fileDialogThreadId;
 		
+		if (fileDialogThreadId != 0) {
+            EnumWindows(enumWindowsToFindAndDestroyFileDialogProc, fileDialogThreadId);
+		}
         if (__isExternalAddress(threadHandleAddress)) {
             HANDLE fileDialogThread = _HWNDVal(threadHandleAddress);
             TerminateThread(fileDialogThread, 0);