diff --git a/ext/pgsql/pgsql.c b/ext/pgsql/pgsql.c
index 4b03bfc3c9df4..c0d4a5117aefc 100644
--- a/ext/pgsql/pgsql.c
+++ b/ext/pgsql/pgsql.c
@@ -3451,11 +3451,13 @@ PHP_FUNCTION(pg_copy_from)
 					if (UNEXPECTED(!tmp)) {
 						return;
 					}
-					zend_string *zquery = zend_string_alloc(ZSTR_LEN(tmp) + 1, false);
+					// we give allocation room for a potential command line `\n` terminator addition
+					zend_string *zquery = zend_string_alloc(ZSTR_LEN(tmp) + 2, false);
 					memcpy(ZSTR_VAL(zquery), ZSTR_VAL(tmp), ZSTR_LEN(tmp) + 1);
 					ZSTR_LEN(zquery) = ZSTR_LEN(tmp);
-					if (ZSTR_LEN(tmp) > 0 && ZSTR_VAL(zquery)[ZSTR_LEN(tmp)]  != '\n') {
+					if (ZSTR_LEN(tmp) > 0 && ZSTR_VAL(zquery)[ZSTR_LEN(tmp) - 1]  != '\n') {
 						ZSTR_VAL(zquery)[ZSTR_LEN(tmp)] = '\n';
+						ZSTR_VAL(zquery)[ZSTR_LEN(tmp) + 1] = '\0';
 						ZSTR_LEN(zquery) ++;
 					}
 					if (PQputCopyData(pgsql, ZSTR_VAL(zquery), ZSTR_LEN(zquery)) != 1) {
diff --git a/ext/pgsql/tests/gh18148.phpt b/ext/pgsql/tests/gh18148.phpt
new file mode 100644
index 0000000000000..6cc2a2542086f
--- /dev/null
+++ b/ext/pgsql/tests/gh18148.phpt
@@ -0,0 +1,24 @@
+--TEST--
+Fix GH-18148 pg_copy_from() command position offset when giving explicit \n terminator
+--EXTENSIONS--
+pgsql
+--SKIPIF--
+<?php
+include("inc/skipif.inc");
+?>
+--FILE--
+<?php
+
+include "inc/config.inc";
+$table_name = "gh18148";
+$db = pg_connect($conn_str);
+pg_query($db, "CREATE TABLE {$table_name} (a integer, b text)");
+var_dump(pg_copy_from( $db, $table_name, [ "1\tone\n" ] ));
+--CLEAN--
+<?php
+include('inc/config.inc');
+$db = pg_connect($conn_str);
+pg_query($db, "DROP TABLE IF EXISTS gh18148 cascade");
+?>
+--EXPECT--
+bool(true)