diff --git a/mozilla/xpcom/threads/plevent.c b/mozilla/xpcom/threads/plevent.c
index 1c46a832b31..893fb00a484 100644
--- a/mozilla/xpcom/threads/plevent.c
+++ b/mozilla/xpcom/threads/plevent.c
@@ -21,6 +21,8 @@
 #endif
 
 #if defined(XP_OS2)
+#define INCL_DOS
+#define INCL_DOSERRORS
 #define INCL_WIN
 #include <os2.h>
 #define DefWindowProc WinDefWindowProc