diff --git a/.changelog/12962.txt b/.changelog/12962.txt
new file mode 100644
index 000000000000..d8a2e2c81a83
--- /dev/null
+++ b/.changelog/12962.txt
@@ -0,0 +1,3 @@
+```release-note:bug
+agent: fixed a panic on startup when the `server.protocol_version` config parameter was set
+```
diff --git a/command/agent/command.go b/command/agent/command.go
index 137eb08ef5d3..880cb0a92e56 100644
--- a/command/agent/command.go
+++ b/command/agent/command.go
@@ -441,7 +441,7 @@ func (c *Command) IsValidConfig(config, cmdConfig *Config) bool {
 	// ProtocolVersion has never been used. Warn if it is set as someone
 	// has probably made a mistake.
 	if config.Server.ProtocolVersion != 0 {
-		c.agent.logger.Warn("Please remove deprecated protocol_version field from config.")
+		c.Ui.Warn("Please remove deprecated protocol_version field from config.")
 	}
 
 	return true