diff --git a/contrib/test_decoding/expected/replorigin.out b/contrib/test_decoding/expected/replorigin.out
index 29a9630c900..14b91e12c10 100644
--- a/contrib/test_decoding/expected/replorigin.out
+++ b/contrib/test_decoding/expected/replorigin.out
@@ -292,6 +292,10 @@ SELECT pg_replication_origin_session_setup('regress_test_decoding: regression_sl
 (1 row)
 
 INSERT INTO target_tbl(data) VALUES ('test data');
+-- invalid proto_version
+SELECT pg_logical_slot_peek_binary_changes('regression_slot', NULL, NULL, 'proto_version', '', 'publication_names', 'pub', 'origin', 'none');
+ERROR:  invalid proto_version
+CONTEXT:  slot "regression_slot", output plugin "pgoutput", in the startup callback
 -- The replayed change will be filtered.
 SELECT count(*) = 0 FROM pg_logical_slot_peek_binary_changes('regression_slot', NULL, NULL, 'proto_version', '4', 'publication_names', 'pub', 'origin', 'none');
  ?column? 
diff --git a/contrib/test_decoding/sql/replorigin.sql b/contrib/test_decoding/sql/replorigin.sql
index 17f2b888238..56f20ec8ea6 100644
--- a/contrib/test_decoding/sql/replorigin.sql
+++ b/contrib/test_decoding/sql/replorigin.sql
@@ -138,6 +138,9 @@ SELECT pg_replication_origin_session_setup('regress_test_decoding: regression_sl
 
 INSERT INTO target_tbl(data) VALUES ('test data');
 
+-- invalid proto_version
+SELECT pg_logical_slot_peek_binary_changes('regression_slot', NULL, NULL, 'proto_version', '', 'publication_names', 'pub', 'origin', 'none');
+
 -- The replayed change will be filtered.
 SELECT count(*) = 0 FROM pg_logical_slot_peek_binary_changes('regression_slot', NULL, NULL, 'proto_version', '4', 'publication_names', 'pub', 'origin', 'none');
 
diff --git a/src/backend/replication/pgoutput/pgoutput.c b/src/backend/replication/pgoutput/pgoutput.c
index 4ecfcbff7ab..0afdb1432ca 100644
--- a/src/backend/replication/pgoutput/pgoutput.c
+++ b/src/backend/replication/pgoutput/pgoutput.c
@@ -316,6 +316,7 @@ parse_output_parameters(List *options, PGOutputData *data)
 		{
 			unsigned long parsed;
 			char	   *endptr;
+			const char *val = strVal(defel->arg);
 
 			if (protocol_version_given)
 				ereport(ERROR,
@@ -324,8 +325,8 @@ parse_output_parameters(List *options, PGOutputData *data)
 			protocol_version_given = true;
 
 			errno = 0;
-			parsed = strtoul(strVal(defel->arg), &endptr, 10);
-			if (errno != 0 || *endptr != '\0')
+			parsed = strtoul(val, &endptr, 10);
+			if (endptr == val || errno != 0 || *endptr != '\0')
 				ereport(ERROR,
 						(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 						 errmsg("invalid proto_version")));