diff --git a/openbsc/src/gprs/sgsn_auth.c b/openbsc/src/gprs/sgsn_auth.c
index d77a021..b83294d 100644
--- a/openbsc/src/gprs/sgsn_auth.c
+++ b/openbsc/src/gprs/sgsn_auth.c
@@ -61,7 +61,7 @@
 	acl = talloc_zero(NULL, struct imsi_acl_entry);
 	if (!acl)
 		return -ENOMEM;
-	strncpy(acl->imsi, imsi, sizeof(acl->imsi));
+	strncpy(acl->imsi, imsi, sizeof(acl->imsi) - 1);
 
 	llist_add(&acl->list, &cfg->imsi_acl);