Index: Modules/bz2module.c
===================================================================
--- Modules/bz2module.c	(revision 51128)
+++ Modules/bz2module.c	(working copy)
@@ -861,6 +861,20 @@
 	int bzerror;
 
 	ACQUIRE_LOCK(self);
+	switch (self->mode) {
+		case MODE_WRITE:
+			break;
+
+		case MODE_CLOSED:
+			PyErr_SetString(PyExc_ValueError,
+					"I/O operation on closed file");
+			goto cleanup;
+
+		default:
+			PyErr_SetString(PyExc_IOError,
+					"file is not ready for writing");
+			goto cleanup;
+	}
 	islist = PyList_Check(seq);
 	if  (!islist) {
 		iter = PyObject_GetIter(seq);
@@ -951,8 +965,12 @@
 	Py_INCREF(Py_None);
 	ret = Py_None;
 
-  error:
+cleanup:
 	RELEASE_LOCK(self);
+	return ret;
+
+error:
+	RELEASE_LOCK(self);
 	Py_XDECREF(list);
   	Py_XDECREF(iter);
 	return ret;