diff --git a/federationsender/internal/perform.go b/federationsender/internal/perform.go
index a56d70ca2..831622efe 100644
--- a/federationsender/internal/perform.go
+++ b/federationsender/internal/perform.go
@@ -91,6 +91,8 @@ func (r *FederationSenderInternalAPI) PerformJoin(
 	var httpErr gomatrix.HTTPError
 	if ok := errors.As(lastErr, &httpErr); ok {
 		httpErr.Message = string(httpErr.Contents)
+		// Clear the wrapped error, else serialising to JSON (in polylith mode) will fail
+		httpErr.WrappedError = nil
 		response.LastError = &httpErr
 	} else {
 		response.LastError = &gomatrix.HTTPError{