diff --git a/cast.hpp b/cast.hpp
index d4806dcb..d7dc8612 100644
--- a/cast.hpp
+++ b/cast.hpp
@@ -70,15 +70,26 @@ namespace util::cast {
 
 
     ///////////////////////////////////////////////////////////////////////////
-    // cast to a smaller type and check that both values are still equal.
+    // cast to a smaller type of the same signedness and realness and assert
+    // that both values are still equal.
     //
     // checks will be compiled out if NDEBUG is defined.
-    template <typename NarrowT, typename WideT>
+    template <
+        typename NarrowT,
+        typename WideT,
+        typename = std::enable_if_t<
+            std::is_arithmetic_v<NarrowT> &&
+            std::is_arithmetic_v<WideT> &&
+            std::is_signed_v<NarrowT> == std::is_signed_v<WideT> &&
+            std::is_floating_point_v<NarrowT> == std::is_floating_point_v<WideT> &&
+            sizeof (NarrowT) <= sizeof (WideT),
+            void
+        >
+    >
     constexpr NarrowT
     narrow (const WideT &val)
     {
         static_assert (sizeof (NarrowT) <= sizeof (WideT));
-        static_assert (std::is_signed_v<NarrowT> == std::is_signed_v<WideT>);
 
 #ifndef NDEBUG
         auto narrow = static_cast<NarrowT> (val);