diff --git a/mailnews/addrbook/src/nsDirPrefs.cpp b/mailnews/addrbook/src/nsDirPrefs.cpp
index 425f6b167fa2..bc0018048cc7 100644
--- a/mailnews/addrbook/src/nsDirPrefs.cpp
+++ b/mailnews/addrbook/src/nsDirPrefs.cpp
@@ -22,6 +22,7 @@
 
 /* directory server preferences (used to be dirprefs.c in 4.x) */
 
+#include <ctype.h>
 #include "nsIPref.h"
 #include "nsVoidArray.h"
 #include "nsIServiceManager.h"