diff --git a/src/lib_json/json_writer.cpp b/src/lib_json/json_writer.cpp
index af5434e..ab3979a 100644
--- a/src/lib_json/json_writer.cpp
+++ b/src/lib_json/json_writer.cpp
@@ -74,17 +74,19 @@ std::string valueToString(double value) {
 #if defined(_MSC_VER) && defined(__STDC_SECURE_LIB__) // Use secure version with
                                                       // visual studio 2005 to
                                                       // avoid warning.
-	size_t len;
+	int len;
   #if defined(WINCE)
 		len = _snprintf(buffer, sizeof(buffer), "%.16g", value);
   #else
 		len = sprintf_s(buffer, sizeof(buffer), "%.16g", value);
   #endif
+                assert(len>=0);
 		fixNumericLocale(buffer, buffer + len);
 #else
   if ( isfinite( value ))
 	{
-		size_t len = snprintf(buffer, sizeof(buffer), "%.16g", value);
+		int len = snprintf(buffer, sizeof(buffer), "%.16g", value);
+                assert(len>=0);
 		fixNumericLocale(buffer, buffer + len);
   }
   else