--- a/toolkit/mozapps/update/updater/updater.cpp
+++ b/toolkit/mozapps/update/updater/updater.cpp
@@ -2751,16 +2751,17 @@ int NS_main(int argc, NS_tchar **argv)
       freeArguments(argc, argv);
       CleanupElevatedMacUpdate(true);
     }
 #endif
     return 1;
   }
   // The directory containing the update information.
   NS_tstrncpy(gPatchDirPath, argv[1], MAXPATHLEN);
+  gPatchDirPath[MAXPATHLEN - 1] = NS_T('\0');
 
   // This check is also performed in workmonitor.cpp since the maintenance
   // service can be called directly.
   if (!IsValidFullPath(argv[2])) {
     WriteStatusFile(INVALID_INSTALL_DIR_PATH_ERROR);
     fprintf(stderr, "The install directory path is not valid for this "  \
             "application (" LOG_S ")\n", argv[2]);
 #ifdef XP_MACOSX