diff --git a/include/fmt/format.h b/include/fmt/format.h
index 6ffae2d9..930dfae8 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -254,8 +254,10 @@ class numeric_limits<fmt::internal::dummy_int> :
   // Portable version of signbit.
   static bool isnegative(double x) {
     using namespace fmt::internal;
-    if (const_check(sizeof(signbit(x)) == sizeof(int)))
+    if (const_check(sizeof(signbit(x)) == sizeof(bool) ||
+                    sizeof(signbit(x)) == sizeof(int))) {
       return signbit(x) != 0;
+    }
     if (x < 0) return true;
     if (!isnotanumber(x)) return false;
     int dec = 0, sign = 0;