diff --git a/src/backend/commands/variable.c b/src/backend/commands/variable.c
index e5ddcda0b4..038da968b6 100644
--- a/src/backend/commands/variable.c
+++ b/src/backend/commands/variable.c
@@ -531,14 +531,16 @@ check_XactIsoLevel(int *newval, void **extra, GucSource source)
 		if (FirstSnapshotSet)
 		{
 			GUC_check_errcode(ERRCODE_ACTIVE_SQL_TRANSACTION);
-			GUC_check_errmsg("SET TRANSACTION ISOLATION LEVEL must be called before any query");
+			GUC_check_errmsg("%s TRANSACTION ISOLATION LEVEL must be called before any query",
+							 (source == PGC_S_OVERRIDE) ? "RESET" : "SET");
 			return false;
 		}
 		/* We ignore a subtransaction setting it to the existing value. */
 		if (IsSubTransaction())
 		{
 			GUC_check_errcode(ERRCODE_ACTIVE_SQL_TRANSACTION);
-			GUC_check_errmsg("SET TRANSACTION ISOLATION LEVEL must not be called in a subtransaction");
+			GUC_check_errmsg("%s TRANSACTION ISOLATION LEVEL must not be called in a subtransaction",
+							 (source == PGC_S_OVERRIDE) ? "RESET" : "SET");
 			return false;
 		}
 		/* Can't go to serializable mode while recovery is still active */
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 4c94f09c64..2ffd6fb844 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -8021,6 +8021,17 @@ set_config_option(const char *name, const char *value,
 					newextra = conf->reset_extra;
 					source = conf->gen.reset_source;
 					context = conf->gen.reset_scontext;
+
+					/*
+					 * Reseted transaction_isolation value can be not
+					 * applicable to current transaction
+					 */
+					if (strcmp(name, "transaction_isolation") == 0)
+					{
+						if (!call_enum_check_hook(conf, &newval, &newextra,
+												  source, elevel))
+							return 0;
+					}
 				}
 
 				if (prohibitValueChange)