diff -r 579141d6e353 Modules/_ctypes/_ctypes.c
--- a/Modules/_ctypes/_ctypes.c	Tue Sep 13 09:38:29 2016 +0200
+++ b/Modules/_ctypes/_ctypes.c	Tue Sep 13 13:31:32 2016 +0300
@@ -463,7 +463,11 @@
     Py_ssize_t offset = 0;
 
     StgDictObject *dict = PyType_stgdict(type);
-    assert (dict);
+    if (!dict) {
+        PyErr_SetString(PyExc_TypeError,
+                        "abstract class");
+        return NULL;
+    }
 
     if (!PyArg_ParseTuple(args, "O|n:from_buffer", &obj, &offset))
         return NULL;
@@ -531,7 +535,11 @@
     Py_ssize_t offset = 0;
     PyObject *result;
     StgDictObject *dict = PyType_stgdict(type);
-    assert (dict);
+    if (!dict) {
+        PyErr_SetString(PyExc_TypeError,
+                        "abstract class");
+        return NULL;
+    }
 
     if (!PyArg_ParseTuple(args, "y*|n:from_buffer", &buffer, &offset))
         return NULL;
@@ -920,6 +928,7 @@
     if (proto) {
         StgDictObject *itemdict = PyType_stgdict(proto);
         const char *current_format;
+        /* PyCPointerType_SetProto has verified proto has a stgdict. */
         assert(itemdict);
         /* If itemdict->format is NULL, then this is a pointer to an
            incomplete type.  We create a generic format string
@@ -966,7 +975,11 @@
     StgDictObject *dict;
 
     dict = PyType_stgdict((PyObject *)self);
-    assert(dict);
+    if (!dict) {
+        PyErr_SetString(PyExc_TypeError,
+                        "abstract class");
+        return NULL;
+    }
 
     if (-1 == PyCPointerType_SetProto(dict, type))
         return NULL;
@@ -992,7 +1005,11 @@
     }
 
     typedict = PyType_stgdict(type);
-    assert(typedict); /* Cannot be NULL for pointer types */
+    if (!typedict) {
+        PyErr_SetString(PyExc_TypeError,
+                        "abstract class");
+        return NULL;
+    }
 
     /* If we expect POINTER(<type>), but receive a <type> instance, accept
        it by calling byref(<type>).
@@ -2057,7 +2074,11 @@
     }
 
     dict = PyType_stgdict(type);
-    assert(dict);
+    if (!dict) {
+        PyErr_SetString(PyExc_TypeError,
+                        "abstract class");
+        return NULL;
+    }
 
     /* I think we can rely on this being a one-character string */
     fmt = _PyUnicode_AsString(dict->proto);
@@ -3151,7 +3172,11 @@
     PyObject *argtypes;
 
     dict = PyType_stgdict((PyObject *)type);
-    assert(dict); /* Cannot be NULL. 'type' is a PyCFuncPtr type. */
+    if (!dict) {
+        PyErr_SetString(PyExc_TypeError,
+                        "abstract class");
+        return 0;
+    }
     argtypes = dict->argtypes;
 
     if (paramflags == NULL || dict->argtypes == NULL)
@@ -4784,7 +4809,7 @@
     }
 
     stgdict = PyObject_stgdict((PyObject *)self);
-    assert(stgdict); /* Cannot be NULL fr pointer instances */
+    assert(stgdict); /* Cannot be NULL for pointer instances */
 
     proto = stgdict->proto;
     assert(proto);
@@ -4812,7 +4837,7 @@
     }
 
     stgdict = PyObject_stgdict((PyObject *)self);
-    assert(stgdict); /* Cannot be NULL fr pointer instances */
+    assert(stgdict); /* Cannot be NULL for pointer instances */
     return PyCData_FromBaseObj(stgdict->proto,
                              (PyObject *)self, 0,
                              *(void **)self->b_ptr);
@@ -4831,7 +4856,7 @@
         return -1;
     }
     stgdict = PyObject_stgdict((PyObject *)self);
-    assert(stgdict); /* Cannot be NULL fr pointer instances */
+    assert(stgdict); /* Cannot be NULL for pointer instances */
     assert(stgdict->proto);
     if (!CDataObject_Check(value)) {
         int res = PyObject_IsInstance(value, stgdict->proto);
diff -r 579141d6e353 Modules/_ctypes/cfield.c
--- a/Modules/_ctypes/cfield.c	Tue Sep 13 09:38:29 2016 +0200
+++ b/Modules/_ctypes/cfield.c	Tue Sep 13 13:31:32 2016 +0300
@@ -205,7 +205,11 @@
 {
     CDataObject *dst;
     char *ptr;
-    assert(CDataObject_Check(inst));
+    if (!CDataObject_Check(inst)) {
+        PyErr_SetString(PyExc_TypeError,
+                        "not a ctype instance");
+        return -1;
+    }
     dst = (CDataObject *)inst;
     ptr = dst->b_ptr + self->offset;
     if (value == NULL) {
@@ -225,7 +229,11 @@
         Py_INCREF(self);
         return (PyObject *)self;
     }
-    assert(CDataObject_Check(inst));
+    if (!CDataObject_Check(inst)) {
+        PyErr_SetString(PyExc_TypeError,
+                        "not a ctype instance");
+        return NULL;
+    }
     src = (CDataObject *)inst;
     return PyCData_get(self->proto, self->getfunc, inst,
                      self->index, self->size, src->b_ptr + self->offset);