diff --git a/xpcom/build/BinaryPath.h b/xpcom/build/BinaryPath.h
index ef3d48f5c4c..264caa80c05 100644
--- a/xpcom/build/BinaryPath.h
+++ b/xpcom/build/BinaryPath.h
@@ -76,7 +76,7 @@ private:
       return NS_ERROR_FAILURE;
 
     snprintf(aResult, MAXPATHLEN, "%s/%s", greHome, "dummy");
-    aResult[MAXPATHLEN] = '\0';
+    aResult[MAXPATHLEN-1] = '\0';
     return NS_OK;
   }