diff --git a/Source/appfat.cpp b/Source/appfat.cpp
index d9c2b5bf7..2d502b933 100644
--- a/Source/appfat.cpp
+++ b/Source/appfat.cpp
@@ -6,7 +6,8 @@ DEVILUTION_BEGIN_NAMESPACE
 
 char sz_error_buf[256];
 BOOL terminating;
-int cleanup_thread_id;
+/** Thread id of the last callee to FreeDlg(). */
+SDL_threadID cleanup_thread_id;
 
 void app_fatal(const char *pszFmt, ...)
 {
diff --git a/Source/appfat.h b/Source/appfat.h
index 08ee83373..2cfb7f5cf 100644
--- a/Source/appfat.h
+++ b/Source/appfat.h
@@ -4,7 +4,6 @@
 
 extern char sz_error_buf[256];
 extern BOOL terminating;
-extern int cleanup_thread_id;
 
 void TriggerBreak();
 char *GetErrorStr(DWORD error_code);