diff --git a/crates/wiggle/test-helpers/examples/tracing.rs b/crates/wiggle/test-helpers/examples/tracing.rs
index 63358a4da6..9930f705c0 100644
--- a/crates/wiggle/test-helpers/examples/tracing.rs
+++ b/crates/wiggle/test-helpers/examples/tracing.rs
@@ -27,6 +27,8 @@ witx_literal: "
     errors: { errno => RichError },
 });
 
+impl_errno!(types::Errno);
+
 /// When the `errors` mapping in witx is non-empty, we need to impl the
 /// types::UserErrorConversion trait that wiggle generates from that mapping.
 impl<'a> types::UserErrorConversion for WasiCtx<'a> {