diff --git a/lib/promscrape/discovery/kubernetes/api_watcher.go b/lib/promscrape/discovery/kubernetes/api_watcher.go
index c78b334f8..d5def2f29 100644
--- a/lib/promscrape/discovery/kubernetes/api_watcher.go
+++ b/lib/promscrape/discovery/kubernetes/api_watcher.go
@@ -190,8 +190,12 @@ func newGroupWatcher(apiServer string, ac *promauth.Config, namespaces []string,
 }
 
 func getGroupWatcher(apiServer string, ac *promauth.Config, namespaces []string, selectors []Selector, proxyURL *url.URL) *groupWatcher {
-	key := fmt.Sprintf("apiServer=%s, namespaces=%s, selectors=%s, proxyURL=%v, authConfig=%s",
-		apiServer, namespaces, selectorsKey(selectors), proxyURL, ac.String())
+	proxyURLStr := "<nil>"
+	if proxyURL != nil {
+		proxyURLStr = proxyURL.String()
+	}
+	key := fmt.Sprintf("apiServer=%s, namespaces=%s, selectors=%s, proxyURL=%s, authConfig=%s",
+		apiServer, namespaces, selectorsKey(selectors), proxyURLStr, ac.String())
 	groupWatchersLock.Lock()
 	gw := groupWatchers[key]
 	if gw == nil {