diff --git a/source/auth/auth_util.c b/source/auth/auth_util.c
index 24b05a5..78b66d0 100644
--- a/source/auth/auth_util.c
+++ b/source/auth/auth_util.c
@@ -867,6 +867,27 @@ NTSTATUS create_token_from_username(TALLOC_CTX *mem_ctx, const char *username,
 		*found_username = talloc_strdup(mem_ctx,
 						pdb_get_username(sam_acct));
 
+		/*
+		 * If the SID from lookup_name() was the guest sid, passdb knows
+		 * about the mapping of guest sid to lp_guestaccount()
+		 * username and will return the unix_pw info for a guest
+		 * user. Use it if it's there, else lookup the *uid details
+		 * using sid_to_uid(). See bug #6291 for details. JRA.
+		 */
+
+		/* We must always assign the *uid. */
+		if (sam_acct->unix_pw != NULL) {
+			/* Probably guest, use what we already have.. */
+			*uid = sam_acct->unix_pw->pw_uid;
+		} else {
+			if (!sid_to_uid(&user_sid, uid)) {
+				DEBUG(1, ("passdb user case, sid_to_uid for %s (%s) failed\n",
+					  username, sid_string_dbg(&user_sid)));
+				result = NT_STATUS_NO_SUCH_USER;
+				goto done;
+			}
+		}
+
 	} else 	if (sid_check_is_in_unix_users(&user_sid)) {
 
 		/* This is a unix user not in passdb. We need to ask nss
@@ -882,9 +903,11 @@ NTSTATUS create_token_from_username(TALLOC_CTX *mem_ctx, const char *username,
 
 	unix_user:
 
+		/* We must always assign the *uid. */
 		if (!sid_to_uid(&user_sid, uid)) {
-			DEBUG(1, ("sid_to_uid for %s (%s) failed\n",
+			DEBUG(1, ("unix_user case, sid_to_uid for %s (%s) failed\n",
 				  username, sid_string_dbg(&user_sid)));
+			result = NT_STATUS_NO_SUCH_USER;
 			goto done;
 		}
 
@@ -937,6 +960,14 @@ NTSTATUS create_token_from_username(TALLOC_CTX *mem_ctx, const char *username,
 
 		uint32 dummy;
 
+		/* We must always assign the *uid. */
+		if (!sid_to_uid(&user_sid, uid)) {
+			DEBUG(1, ("winbindd case, sid_to_uid for %s (%s) failed\n",
+				  username, sid_string_dbg(&user_sid)));
+			result = NT_STATUS_NO_SUCH_USER;
+			goto done;
+		}
+
 		num_group_sids = 1;
 		group_sids = TALLOC_ARRAY(tmp_ctx, DOM_SID, num_group_sids);
 		if (group_sids == NULL) {