diff --git a/federationsender/api/api.go b/federationsender/api/api.go
index 655d1d103..adc3b34cd 100644
--- a/federationsender/api/api.go
+++ b/federationsender/api/api.go
@@ -30,7 +30,7 @@ type FederationClientError struct {
 }
 
 func (e *FederationClientError) Error() string {
-	return fmt.Sprintf("%s - (retry_after=%d, blacklisted=%v)", e.Err, e.RetryAfter, e.Blacklisted)
+	return fmt.Sprintf("%s - (retry_after=%s, blacklisted=%v)", e.Err, e.RetryAfter.String(), e.Blacklisted)
 }
 
 // FederationSenderInternalAPI is used to query information from the federation sender.