diff --git a/Doc/library/mmap.rst b/Doc/library/mmap.rst
index b74a823..027d013 100644
--- a/Doc/library/mmap.rst
+++ b/Doc/library/mmap.rst
@@ -263,7 +263,9 @@ To map anonymous memory, -1 should be passed as the fileno along with the length
    .. method:: write(bytes)
 
       Write the bytes in *bytes* into memory at the current position of the
-      file pointer; the file position is updated to point after the bytes that
+      file pointer and return the number of bytes written (never less than
+      len(bytes), since if the write fails a :exc:`ValueError` will be
+      raised). The file position is updated to point after the bytes that
       were written. If the mmap was created with :const:`ACCESS_READ`, then
       writing to it will raise a :exc:`TypeError` exception.
 
diff --git a/Lib/test/test_mmap.py b/Lib/test/test_mmap.py
index 0f25742..4851f8c 100644
--- a/Lib/test/test_mmap.py
+++ b/Lib/test/test_mmap.py
@@ -32,9 +32,9 @@ class MmapTests(unittest.TestCase):
         f = open(TESTFN, 'bw+')
         try:
             # Write 2 pages worth of data to the file
-            f.write(b'\0'* PAGESIZE)
-            f.write(b'foo')
-            f.write(b'\0'* (PAGESIZE-3) )
+            self.assertEqual(f.write(b'\0'* PAGESIZE), PAGESIZE)
+            self.assertEqual(f.write(b'foo'), 3)
+            self.assertEqual(f.write(b'\0'* (PAGESIZE-3) ), PAGESIZE - 3)
             f.flush()
             m = mmap.mmap(f.fileno(), 2 * PAGESIZE)
         finally:
diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index bb98a99..28a6b83 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -408,7 +408,7 @@ mmap_write_method(mmap_object *self,
     self->pos = self->pos + data.len;
     PyBuffer_Release(&data);
     Py_INCREF(Py_None);
-    return Py_None;
+    return PyLong_FromSsize_t(data.len);
 }
 
 static PyObject *