.

bigfile/_bigfile.c

@@ -958,6 +958,7 @@ pyfile_new(PyTypeObject *type, PyObject *args, PyObject *kw)
 {
     PyBigFile    *self;
     PyObject     *blkmmapper;
+    bigfile_ops  *blkmmap_ops = NULL;
 
     blkmmapper = PyObject_GetAttrString((PyObject*)type, "blkmmapper");
     PyErr_Clear(); /* GetAttr raises exception if there is no attribute */
@@ -967,6 +968,12 @@ pyfile_new(PyTypeObject *type, PyObject *args, PyObject *kw)
             return PyErr_Format(PyExc_TypeError,
                 "%s: .blkmmapper is not a valid pycapsule with mmap methods", type->tp_name);
         }
+
+        blkmmap_ops = PyCapsule_GetPointer(blkmmapper, "wendelin.bigfile.IBlkMMapper");
+        if (blkmmap_ops == NULL) { /* just in case - must not fail */
+            Py_DECREF(blkmmapper);
+            return NULL;
+        }
     }
 
     self = (PyBigFile *)PyType_GenericNew(type, args, kw);