diff --git a/src/backend/libpq/auth-oauth.c b/src/backend/libpq/auth-oauth.c
index b769931ca4f..fb830065fba 100644
--- a/src/backend/libpq/auth-oauth.c
+++ b/src/backend/libpq/auth-oauth.c
@@ -863,20 +863,8 @@ check_oauth_validator(HbaLine *hbaline, int elevel, char **err_msg)
 
 	*err_msg = NULL;
 
-	if (oauth_validator_libraries_string[0] == '\0')
-	{
-		ereport(elevel,
-				errcode(ERRCODE_CONFIG_FILE_ERROR),
-				errmsg("parameter \"%s\" must be set for authentication method \"%s\"",
-					   "oauth_validator_libraries", "oauth"),
-				errcontext("line %d of configuration file \"%s\"",
-						   line_num, file_name));
-		*err_msg = psprintf("parameter \"%s\" must be set for authentication method \"%s\"",
-							"oauth_validator_libraries", "oauth");
-		return false;
-	}
-
 	/* SplitDirectoriesString needs a modifiable copy */
+	Assert(oauth_validator_libraries_string != NULL);
 	rawstring = pstrdup(oauth_validator_libraries_string);
 
 	if (!SplitDirectoriesString(rawstring, ',', &elemlist))
@@ -891,6 +879,19 @@ check_oauth_validator(HbaLine *hbaline, int elevel, char **err_msg)
 		goto done;
 	}
 
+	if (elemlist == NIL)
+	{
+		ereport(elevel,
+				errcode(ERRCODE_CONFIG_FILE_ERROR),
+				errmsg("parameter \"%s\" must be set for authentication method \"%s\"",
+					   "oauth_validator_libraries", "oauth"),
+				errcontext("line %d of configuration file \"%s\"",
+						   line_num, file_name));
+		*err_msg = psprintf("parameter \"%s\" must be set for authentication method \"%s\"",
+							"oauth_validator_libraries", "oauth");
+		goto done;
+	}
+
 	if (!hbaline->oauth_validator)
 	{
 		if (elemlist->length == 1)
diff --git a/src/test/modules/oauth_validator/t/001_server.pl b/src/test/modules/oauth_validator/t/001_server.pl
index 8941a355423..fb76c69c1ca 100644
--- a/src/test/modules/oauth_validator/t/001_server.pl
+++ b/src/test/modules/oauth_validator/t/001_server.pl
@@ -134,6 +134,24 @@ is( $contents,
 3|oauth|\{issuer=$issuer/param,"scope=openid postgres",validator=validator\}},
 	"pg_hba_file_rules recreates OAuth HBA settings");
 
+# An all-whitespace library list parses as an empty list.  Reject it without
+# crashing the postmaster during HBA reload.
+$node->append_conf('postgresql.conf', "oauth_validator_libraries = '   '");
+$node->reload;
+$log_start = $node->wait_for_log(
+	qr/parameter "oauth_validator_libraries" must be set for authentication/,
+	$log_start);
+$bgconn->query_safe('SELECT 1');
+
+$node->append_conf('postgresql.conf',
+	"oauth_validator_libraries = 'validator'");
+$node->reload;
+$log_start = $node->wait_for_log(
+	qr/parameter "oauth_validator_libraries" changed to "validator"/,
+	$log_start);
+is($bgconn->query_safe('SHOW oauth_validator_libraries'),
+	'validator', 'oauth_validator_libraries restored');
+
 {
 	# Make sure PGOAUTHDEBUG=UNSAFE doesn't disable certificate verification.
 	local $ENV{PGOAUTHDEBUG} = "UNSAFE";