diff --git a/include/utils.hpp b/include/utils.hpp
index f40f9c3c1..cc826dcd3 100644
--- a/include/utils.hpp
+++ b/include/utils.hpp
@@ -21,7 +21,7 @@ namespace hex {
 
     template<typename ... Args>
     inline std::string format(const std::string &format, Args ... args) {
-        size_t size = snprintf( nullptr, 0, format.c_str(), args ... );
+        ssize_t size = snprintf( nullptr, 0, format.c_str(), args ... );
 
         if( size <= 0 )
             return "";