diff --git a/doc/src/sgml/protocol.sgml b/doc/src/sgml/protocol.sgml
index cba6661..24c57fb 100644
--- a/doc/src/sgml/protocol.sgml
+++ b/doc/src/sgml/protocol.sgml
@@ -2192,7 +2192,7 @@ psql "dbname=postgres replication=database" -c "IDENTIFY_SYSTEM;"
        </varlistentry>
       </variablelist>
 
-      <para>The following option is supported:</para>
+      <para>The following options are supported:</para>
 
       <variablelist>
        <varlistentry>
diff --git a/src/backend/access/transam/twophase.c b/src/backend/access/transam/twophase.c
index f710030..b426f0b 100644
--- a/src/backend/access/transam/twophase.c
+++ b/src/backend/access/transam/twophase.c
@@ -2689,7 +2689,7 @@ LookupGXact(const char *gid, XLogRecPtr prepare_end_lsn,
  * Return the GID in the supplied buffer.
  */
 void
-TwoPhaseTransactionGid(Oid subid, TransactionId xid, char *gid, int szgid)
+TwoPhaseTransactionGid(Oid subid, TransactionId xid, char *gid_res, int szgid)
 {
 	Assert(subid != InvalidRepOriginId);
 
@@ -2698,7 +2698,7 @@ TwoPhaseTransactionGid(Oid subid, TransactionId xid, char *gid, int szgid)
 				(errcode(ERRCODE_PROTOCOL_VIOLATION),
 				 errmsg_internal("invalid two-phase transaction ID")));
 
-	snprintf(gid, szgid, "pg_gid_%u_%u", subid, xid);
+	snprintf(gid_res, szgid, "pg_gid_%u_%u", subid, xid);
 }
 
 /*
@@ -2710,20 +2710,26 @@ static bool
 IsTwoPhaseTransactionGidForSubid(Oid subid, char *gid)
 {
 	int			ret;
-	Oid			subid_written;
-	TransactionId xid;
+	Oid			subid_from_gid;
+	TransactionId xid_from_gid;
 	char		gid_generated[GIDSIZE];
 
-	ret = sscanf(gid, "pg_gid_%u_%u", &subid_written, &xid);
+	/* Extract the subid and xid from the given GID. */
+	ret = sscanf(gid, "pg_gid_%u_%u", &subid_from_gid, &xid_from_gid);
 
-	/* Return false if the given GID has different format */
-	if (ret != 2 || subid != subid_written)
+	/*
+	 * Check that the given GID has expected format, and at least the subid
+	 * matches.
+	 */
+	if (ret != 2 || subid != subid_from_gid)
 		return false;
 
-	/* Construct the format GID based on the got xid */
-	TwoPhaseTransactionGid(subid, xid, gid_generated, sizeof(gid));
-
-	/* ...And check whether the given GID is exact same as the format GID */
+	/*
+	 * Reconstruct a tmp GID based on the subid and xid extracted from
+	 * the given GID. Check that the tmp GID and the given GID match.
+	 */
+	TwoPhaseTransactionGid(subid_from_gid, xid_from_gid,
+						   gid_generated, sizeof(gid_generated));
 	return strcmp(gid, gid_generated) == 0;
 }