diff --git a/utils/funcexp/jsonhelpers.cpp b/utils/funcexp/jsonhelpers.cpp
index 1658e8f78..f6bab550c 100644
--- a/utils/funcexp/jsonhelpers.cpp
+++ b/utils/funcexp/jsonhelpers.cpp
@@ -31,7 +31,7 @@ bool appendEscapedJS(string& ret, const CHARSET_INFO* retCS, const utils::NullSt
   const int jsLen = js.length();
   const char* rawJS = js.str();
   int strLen = jsLen * 12 * jsCS->mbmaxlen / jsCS->mbminlen;
-  char* buf = (char*)alloca(strLen);
+  char* buf = (char*)alloca(strLen + 1);
   if ((strLen = json_escape(retCS, (const uchar*)rawJS, (const uchar*)rawJS + jsLen, jsCS, (uchar*)buf,
                             (uchar*)buf + strLen)) >= 0)
   {