diff --git a/src/lib_json/json_value.cpp b/src/lib_json/json_value.cpp
index aa2b744..a176787 100644
--- a/src/lib_json/json_value.cpp
+++ b/src/lib_json/json_value.cpp
@@ -24,7 +24,11 @@ static int msvc_pre1900_c99_vsnprintf(char* outBuf, size_t size,
                                       const char* format, va_list ap) {
   int count = -1;
   if (size != 0)
-    count = _vsnprintf_s(outBuf, size, _TRUNCATE, format, ap);
+    {
+      if (!outBuf)
+        return 0;
+      count = _vsnprintf_s(outBuf, size, _TRUNCATE, format, ap);
+    }
   if (count == -1)
     count = _vscprintf(format, ap);
   return count;