--- ./internal/kafka/franz_client.go.bak	2026-08-23 06:42:46.645316995 -0500
+++ ./internal/kafka/franz_client.go	2026-08-23 06:43:08.092257543 -0500
@@ -310,6 +310,9 @@
 	if clientCfg.ProtocolVersion != "" {
 		keyVersions := make(map[string]any)
 		versions := kversion.FromString(clientCfg.ProtocolVersion)
+		if versions == nil {
+			return nil, fmt.Errorf("unsupported kafka protocol_version %q; try a version like 3.6 or 3.8", clientCfg.ProtocolVersion)
+		}
 		versions.EachMaxKeyVersion(func(k, v int16) {
 			name := kmsg.NameForKey(k)
 			keyVersions[name] = v