diff --git a/cmd/tailscale/cli/serve_v2.go b/cmd/tailscale/cli/serve_v2.go
index 3af0c0403..08a1ff906 100644
--- a/cmd/tailscale/cli/serve_v2.go
+++ b/cmd/tailscale/cli/serve_v2.go
@@ -713,7 +713,7 @@ func (e *serveEnv) applyTCPServe(sc *ipn.ServeConfig, dnsName string, srcType se
 
 	// TODO: needs to account for multiple configs from foreground mode
 	if sc.IsServingWeb(srcPort, dnsName) {
-		return fmt.Errorf("cannot serve TCP; already serving web on %d", srcPort)
+		return fmt.Errorf("cannot serve TCP; already serving web on %d for %s", srcPort, dnsName)
 	}
 
 	sc.SetTCPForwarding(srcPort, dstURL.Host, terminateTLS, dnsName)