diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index ba27b274c967..0b1808e3a912 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -322,6 +322,7 @@ static int addr_cnt;
 #define GSM1_ESCAPE_BITS	0x20
 #define XON			0x11
 #define XOFF			0x13
+#define ISO_IEC_646_MASK	0x7F
 
 static const struct tty_port_operations gsm_port_ops;
 
@@ -531,7 +532,8 @@ static int gsm_stuff_frame(const u8 *input, u8 *output, int len)
 	int olen = 0;
 	while (len--) {
 		if (*input == GSM1_SOF || *input == GSM1_ESCAPE
-		    || *input == XON || *input == XOFF) {
+		    || (*input & ISO_IEC_646_MASK) == XON
+		    || (*input & ISO_IEC_646_MASK) == XOFF) {
 			*output++ = GSM1_ESCAPE;
 			*output++ = *input++ ^ GSM1_ESCAPE_BITS;
 			olen++;