diff --git a/ext/opcache/jit/zend_jit_helpers.c b/ext/opcache/jit/zend_jit_helpers.c
index 5a68e15588e..95ff48dd4f9 100644
--- a/ext/opcache/jit/zend_jit_helpers.c
+++ b/ext/opcache/jit/zend_jit_helpers.c
@@ -1135,7 +1135,8 @@ try_string_offset:
 				goto try_string_offset;
 			default:
 				zend_jit_illegal_string_offset(dim);
-				break;
+				ZVAL_NULL(result);
+				return;
 		}
 
 		offset = zval_get_long_func(dim, /* is_strict */ false);
diff --git a/ext/opcache/tests/jit/gh12748.phpt b/ext/opcache/tests/jit/gh12748.phpt
new file mode 100644
index 00000000000..d7580fdb9ab
--- /dev/null
+++ b/ext/opcache/tests/jit/gh12748.phpt
@@ -0,0 +1,35 @@
+--TEST--
+GH-12748: Function JIT emits "could not convert to int" warning at the same time as invalid offset Error
+--INI--
+opcache.enable=1
+opcache.enable_cli=1
+--FILE--
+<?php
+$container = "string";
+// Is
+try {
+    echo "isset():\n";
+    var_dump(isset($container[new stdClass()]));
+} catch (\Throwable $e) {
+    echo $e->getMessage(), "\n";
+}
+try {
+    echo "empty():\n";
+    var_dump(empty($container[new stdClass()]));
+} catch (\Throwable $e) {
+    echo $e->getMessage(), "\n";
+}
+try {
+    echo "Coalesce():\n";
+    var_dump($container[new stdClass()] ?? 'default');
+} catch (\Throwable $e) {
+    echo $e->getMessage(), "\n";
+}
+?>
+--EXPECT--
+isset():
+bool(false)
+empty():
+bool(true)
+Coalesce():
+Cannot access offset of type stdClass on string