diff --git a/CmdMessenger.cpp b/CmdMessenger.cpp
index cbb4ee7..5386863 100644
--- a/CmdMessenger.cpp
+++ b/CmdMessenger.cpp
@@ -299,7 +299,7 @@ void CmdMessenger::sendCmdEscArg(char* arg)
  * Send formatted argument.
  *  Note that floating points are not supported and resulting string is limited to 128 chars
  */
-void CmdMessenger::sendCmdfArg(char *fmt, ...)
+void CmdMessenger::sendCmdfArg(const char * const fmt, ...)
 {
 	const int maxMessageSize = 128;
 	if (startCommand) {
@@ -675,4 +675,4 @@ void CmdMessenger::printSci(double f, unsigned int digits)
 	char output[16];
 	sprintf(output, format, whole, part, exponent);
 	comms->print(output);
-}
\ No newline at end of file
+}
diff --git a/CmdMessenger.h b/CmdMessenger.h
index 1072bd7..20cb9f3 100644
--- a/CmdMessenger.h
+++ b/CmdMessenger.h
@@ -220,7 +220,7 @@ class CmdMessenger
 
 	void sendCmdStart(byte cmdId);
 	void sendCmdEscArg(char *arg);
-	void sendCmdfArg(char *fmt, ...);
+	void sendCmdfArg(const char * const fmt, ...);
 	bool sendCmdEnd(bool reqAc = false, byte ackCmdId = 1, unsigned int timeout = DEFAULT_TIMEOUT);
 
 	/**