diff --git a/include/osmocom/gsm/gsm0480.h b/include/osmocom/gsm/gsm0480.h
index d6626d6..f6c3734 100644
--- a/include/osmocom/gsm/gsm0480.h
+++ b/include/osmocom/gsm/gsm0480.h
@@ -8,7 +8,7 @@
 #define MAX_LEN_USSD_STRING	31
 
 struct ussd_request {
-	char text[MAX_LEN_USSD_STRING + 1];
+	uint8_t text[MAX_LEN_USSD_STRING + 1];
 	uint8_t transaction_id;
 	uint8_t invoke_id;
 };