Add missing message.
diff --git a/gen/src/write.rs b/gen/src/write.rs
index 70620ce..3c059a7 100644
--- a/gen/src/write.rs
+++ b/gen/src/write.rs
@@ -380,7 +380,7 @@
 fn check_enum(out: &mut OutFile, enm: &Enum) {
     writeln!(
         out,
-        "static_assert(sizeof({}) == sizeof(uint32_t));",
+        "static_assert(sizeof({}) == sizeof(uint32_t), \"incorrect size\");",
         enm.ident
     );
     let mut prev_discriminant = None;