diff --git a/src/json.hpp b/src/json.hpp
index b781b98c..b15470b6 100644
--- a/src/json.hpp
+++ b/src/json.hpp
@@ -6660,7 +6660,7 @@ class serializer
             return;
         }
 
-        const bool is_negative = x < 0;
+        const bool is_negative = std::is_same<NumberType, number_integer_t>::value and (x < 0);
         std::size_t i = 0;
 
         // spare 1 byte for '\0'