1 files changed, 9 insertions, 0 deletions
diff --git a/Modules/itertoolsmodule.c b/Modules/itertoolsmodule.c
index 04076fd..edd21be 100644
--- a/Modules/itertoolsmodule.c
+++ b/Modules/itertoolsmodule.c
@@ -314,6 +314,7 @@ typedef struct {
     PyObject_HEAD
     PyObject *it;
     int numread;
+    int running;
     PyObject *nextlink;
     PyObject *(values[LINKCELLS]);
 } teedataobject;
@@ -336,6 +337,7 @@ teedataobject_new(PyObject *it)
     if (tdo == NULL)
         return NULL;
 
+    tdo->running = 0;
     tdo->numread = 0;
     tdo->nextlink = NULL;
     Py_INCREF(it);
@@ -364,7 +366,14 @@ teedataobject_getitem(teedataobject *tdo, int i)
     else {
         /* this is the lead iterator, so fetch more data */
         assert(i == tdo->numread);
+        if (tdo->running) {
+            PyErr_SetString(PyExc_RuntimeError,
+                            "cannot re-enter the tee iterator");
+            return NULL;
+        }
+        tdo->running = 1;
         value = PyIter_Next(tdo->it);
+        tdo->running = 0;
         if (value == NULL)
             return NULL;
         tdo->numread++;