diff --git a/include/boost/math/special_functions/fast_float_distance.hpp b/include/boost/math/special_functions/fast_float_distance.hpp
index 17ddbd3373..e350c8efd2 100644
--- a/include/boost/math/special_functions/fast_float_distance.hpp
+++ b/include/boost/math/special_functions/fast_float_distance.hpp
@@ -63,7 +63,7 @@ boost::multiprecision::int128_type fast_float_distance(boost::multiprecision::fl
         BOOST_MATH_THROW_EXCEPTION(std::domain_error("Both arguments to fast_float_distnace must be finite"));
     }
 
-    static_assert(sizeof(boost::multiprecision::int128_type) == sizeof(boost::multiprecision::float128_type));
+    static_assert(sizeof(boost::multiprecision::int128_type) == sizeof(boost::multiprecision::float128_type), "float128 is the wrong size");
 
     boost::multiprecision::int128_type ai;
     boost::multiprecision::int128_type bi;