diff --git a/octoprint/util/comm.py b/octoprint/util/comm.py
index 3161084..00d63fe 100644
--- a/octoprint/util/comm.py
+++ b/octoprint/util/comm.py
@@ -461,6 +461,7 @@ class MachineCom(object):
 		self.close()
 	
 	def _sendCommand(self, cmd):
+		cmd = cmd.upper()
 		if self._serial is None:
 			return
 		if 'M109' in cmd or 'M190' in cmd: