diff --git a/src/hlr.c b/src/hlr.c
index e66444d..ac6afc3 100644
--- a/src/hlr.c
+++ b/src/hlr.c
@@ -401,7 +401,7 @@
 	/* TODO: Set subscriber tracing = deactive in VLR/SGSN */
 
 #if 0
-	/* Cancel in old VLR/SGSN, if new VLR/SGSN differs from old */
+	/* Cancel in old VLR/SGSN, if new VLR/SGSN differs from old (FIXME: OS#4491) */
 	if (luop->is_ps == false &&
 	    strcmp(subscr->vlr_number, vlr_number)) {
 		lu_op_tx_cancel_old(luop);
diff --git a/src/luop.c b/src/luop.c
index 1a2f854..e63ba91 100644
--- a/src/luop.c
+++ b/src/luop.c
@@ -194,7 +194,7 @@
 	lu_op_free(luop);
 }
 
-/*! Send Cancel Location to old VLR/SGSN */
+/*! Send Cancel Location to old VLR/SGSN (FIXME: OS#4491) */
 void lu_op_tx_cancel_old(struct lu_operation *luop)
 {
 	struct osmo_gsup_message gsup;