diff --git a/Modules/_json.c b/Modules/_json.c
index ec980c93fa1..4bc585dc2ab 100644
--- a/Modules/_json.c
+++ b/Modules/_json.c
@@ -975,10 +975,11 @@ scan_once_unicode(PyScannerObject *s, PyObject *pystr, Py_ssize_t idx, Py_ssize_
     kind = PyUnicode_KIND(pystr);
     length = PyUnicode_GET_LENGTH(pystr);
 
-    if (idx < 0)
-        /* Compatibility with Python version. */
-        idx += length;
-    if (idx < 0 || idx >= length) {
+    if (idx < 0) {
+        PyErr_SetString(PyExc_ValueError, "idx cannot be negative");
+        return NULL;
+    }
+    if (idx >= length) {
         PyErr_SetNone(PyExc_StopIteration);
         return NULL;
     }