fixed compiler warning
diff --git a/src/lib_json/json_value.cpp b/src/lib_json/json_value.cpp
index 0fb735f..e93e346 100644
--- a/src/lib_json/json_value.cpp
+++ b/src/lib_json/json_value.cpp
@@ -151,7 +151,7 @@
    if ( comment_ )

       valueAllocator()->releaseStringValue( comment_ );

    JSON_ASSERT( text );

-   JSON_ASSERT_MESSAGE( text[0]==NULL || text[0]=='/', "Comments must start with /");

+   JSON_ASSERT_MESSAGE( text[0]=='\0' || text[0]=='/', "Comments must start with /");

    // It seems that /**/ style comments are acceptable as well.

    comment_ = valueAllocator()->duplicateStringValue( text );

 }