diff --git a/mozilla/intl/uconv/ucvcn/nsISO2022CNToUnicode.h b/mozilla/intl/uconv/ucvcn/nsISO2022CNToUnicode.h
index f429865a6b9..bac23636cf6 100644
--- a/mozilla/intl/uconv/ucvcn/nsISO2022CNToUnicode.h
+++ b/mozilla/intl/uconv/ucvcn/nsISO2022CNToUnicode.h
@@ -42,6 +42,7 @@
 #include "nsUCSupport.h"
 
 #define MBYTE       0x8e
+#undef PMASK
 #define PMASK       0xa0
 
 #define SI          0x0f