diff --git a/src/stratifier.c b/src/stratifier.c
index 91c9bb84..1c2d673f 100644
--- a/src/stratifier.c
+++ b/src/stratifier.c
@@ -1144,6 +1144,9 @@ static uint64_t disconnected_sessionid_exists(sdata_t *sdata, const char *sessio
 	if (slen < 1 || slen > 8)
 		goto out;
 
+	if (!validhex(sessionid))
+		goto out;
+
 	/* Number is in BE but we don't swap either of them */
 	hex2bin(&enonce1_64, sessionid, slen);