diff --git a/src/modem.c b/src/modem.c
index 70d6b81..6f7caf2 100644
--- a/src/modem.c
+++ b/src/modem.c
@@ -484,7 +484,8 @@ int gsm_modem_send_message(GSMModem * gsmm, char const * number,
 				GSM_ERROR_MESSAGE_SEND_FAILED, NULL);
 	}
 	snprintf(buf2, len2, "%s%02lX%s%s%s%s%02lX%s\x1a", cmd2,
-			number[0] == '+' ? strlen(number) - 1 : strlen(number),
+			(unsigned long)((number[0] == '+')
+				? strlen(number) - 1 : strlen(number)),
 			addr, pid, dcs, vp, (unsigned long)strlen(text), sept);
 	snprintf(buf1, len1, "%s%lu", cmd1, (len2 - 1) / 2);
 	free(addr);