1 files changed, 6 insertions, 0 deletions
diff --git a/contrib/tools/python3/Modules/_csv.c b/contrib/tools/python3/Modules/_csv.c
index 0b7ff2575fc..755d005f4d8 100644
--- a/contrib/tools/python3/Modules/_csv.c
+++ b/contrib/tools/python3/Modules/_csv.c
@@ -957,6 +957,12 @@ Reader_iternext(ReaderObj *self)
             Py_DECREF(lineobj);
             return NULL;
         }
+        if (self->fields == NULL) {
+            PyErr_SetString(module_state->error_obj,
+                            "iterator has already advanced the reader");
+            Py_DECREF(lineobj);
+            return NULL;
+        }
         ++self->line_num;
         kind = PyUnicode_KIND(lineobj);
         data = PyUnicode_DATA(lineobj);