diff --git a/include/osmocom/gsm/gsup.h b/include/osmocom/gsm/gsup.h
index 29ea11a..9a583aa 100644
--- a/include/osmocom/gsm/gsup.h
+++ b/include/osmocom/gsm/gsup.h
@@ -232,7 +232,7 @@
 /*! parsed/decoded GSUP protocol message */
 struct osmo_gsup_message {
 	enum osmo_gsup_message_type	message_type;
-	char				imsi[GSM23003_IMSI_MAX_DIGITS+2];
+	char				imsi[OSMO_IMSI_BUF_SIZE];
 	enum gsm48_gmm_cause		cause;
 	enum osmo_gsup_cancel_type	cancel_type;
 	int				pdp_info_compl;
diff --git a/include/osmocom/gsm/protocol/gsm_23_003.h b/include/osmocom/gsm/protocol/gsm_23_003.h
index babd0f4..be1b157 100644
--- a/include/osmocom/gsm/protocol/gsm_23_003.h
+++ b/include/osmocom/gsm/protocol/gsm_23_003.h
@@ -5,6 +5,9 @@
 /* Chapter 2.2 */
 #define GSM23003_IMSI_MAX_DIGITS	15
 #define GSM23003_IMSI_MIN_DIGITS	6
+/*! The char[] buffer size to completely contain an IMSI including the optional checksum digit as well as the
+ * terminating nul character. */
+#define OSMO_IMSI_BUF_SIZE (GSM23003_IMSI_MAX_DIGITS+2)
 /* Chapter 2.4 */
 #define GSM23003_TMSI_NUM_BYTES		4
 /* Chapter 2.5 */