diff -r 6c9f4c22fd81 Modules/_io/textio.c
--- a/Modules/_io/textio.c	Sat Feb 02 15:08:52 2013 -0800
+++ b/Modules/_io/textio.c	Sun Feb 03 10:37:22 2013 +0200
@@ -269,8 +269,9 @@
         return NULL;
 
     if (!PyUnicode_Check(output)) {
-        PyErr_SetString(PyExc_TypeError,
-                        "decoder should return a string result");
+        PyErr_Format(PyExc_TypeError,
+                     "decoder should return a string result, not '%.200s'",
+                     Py_TYPE(output)->tp_name);
         goto error;
     }
 
@@ -1454,7 +1455,13 @@
     Py_DECREF(chunk_size);
     if (input_chunk == NULL)
         goto fail;
-    assert(PyBytes_Check(input_chunk));
+    if (!PyBytes_Check(input_chunk)) {
+        PyErr_Format(PyExc_IOError,
+                     "underlying %s() should have returned a bytes object, "
+                     "not '%.200s'", (self->has_read1 ? "read1": "read"),
+                     Py_TYPE(input_chunk)->tp_name);
+        goto fail;
+    }
 
     eof = (PyBytes_Size(input_chunk) == 0);
 
@@ -1481,7 +1488,14 @@
         PyObject *next_input = PyNumber_Add(dec_buffer, input_chunk);
         if (next_input == NULL)
             goto fail;
-        assert (PyBytes_Check(next_input));
+        if (!PyBytes_Check(next_input)) {
+            PyErr_Format(PyExc_TypeError,
+                         "decoder getstate() should have returned a bytes "
+                         "object, not '%.200s'",
+                         Py_TYPE(next_input)->tp_name);
+            Py_DECREF(next_input);
+            goto fail;
+        }
         Py_DECREF(dec_buffer);
         Py_CLEAR(self->snapshot);
         self->snapshot = Py_BuildValue("NN", dec_flags, next_input);
@@ -2123,7 +2137,14 @@
         if (input_chunk == NULL)
             goto fail;
 
-        assert (PyBytes_Check(input_chunk));
+        if (!PyBytes_Check(input_chunk)) {
+            PyErr_Format(PyExc_IOError,
+                         "underlying read() should have returned a bytes "
+                         "object, not '%.200s'",
+                         Py_TYPE(input_chunk)->tp_name);
+            Py_DECREF(input_chunk);
+            goto fail;
+        }
 
         self->snapshot = Py_BuildValue("iN", cookie.dec_flags, input_chunk);
         if (self->snapshot == NULL) {
@@ -2259,7 +2280,13 @@
             self->decoder, "decode", "y#", input, 1);
         if (decoded == NULL)
             goto fail;
-        assert (PyUnicode_Check(decoded));
+        if (!PyUnicode_Check(decoded)) {
+            PyErr_Format(PyExc_TypeError,
+                         "decoder should return a string result, not '%.200s'",
+                         Py_TYPE(decoded)->tp_name);
+            Py_DECREF(decoded);
+            goto fail;
+        }
         chars_decoded += PyUnicode_GET_SIZE(decoded);
         Py_DECREF(decoded);
 
@@ -2293,7 +2320,13 @@
             self->decoder, "decode", "yi", "", /* final = */ 1);
         if (decoded == NULL)
             goto fail;
-        assert (PyUnicode_Check(decoded));
+        if (!PyUnicode_Check(decoded)) {
+            PyErr_Format(PyExc_TypeError,
+                         "decoder should return a string result, not '%.200s'",
+                         Py_TYPE(decoded)->tp_name);
+            Py_DECREF(decoded);
+            goto fail;
+        }
         chars_decoded += PyUnicode_GET_SIZE(decoded);
         Py_DECREF(decoded);
         cookie.need_eof = 1;