diff --git a/pySim/transport/modem_atcmd.py b/pySim/transport/modem_atcmd.py
index ecf463c..eef38cb 100644
--- a/pySim/transport/modem_atcmd.py
+++ b/pySim/transport/modem_atcmd.py
@@ -99,6 +99,9 @@
 		pass # Nothing to do really ...
 
 	def _send_apdu_raw(self, pdu):
+		# Make sure pdu has upper case hex digits [A-F]
+		pdu = pdu.upper()
+
 		# Prepare the command as described in 8.17
 		cmd = 'AT+CSIM=%d,\"%s\"' % (len(pdu), pdu)