diff --git a/Lib/test/test_mmap.py b/Lib/test/test_mmap.py
index 712378b..e2dcb24 100644
--- a/Lib/test/test_mmap.py
+++ b/Lib/test/test_mmap.py
@@ -417,6 +417,17 @@ class MmapTests(unittest.TestCase):
             m[x] = b
             self.assertEqual(m[x], b)
 
+    def test_read_all(self):
+        # With no parameters, reads all
+        m = mmap.mmap(-1, 16)
+        m.write(bytes(range(16)))
+        m.seek(0)
+        self.assertEqual(m.read(), bytes(range(16)))
+        m.seek(8)
+        self.assertEqual(m.read(), bytes(range(8, 16)))
+        m.seek(16)
+        self.assertEqual(m.read(), b'')
+
     def test_extended_getslice(self):
         # Test extended slicing by comparing with list slicing.
         s = bytes(reversed(range(256)))
diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index 36ca67d..007a205 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -244,11 +244,11 @@ static PyObject *
 mmap_read_method(mmap_object *self,
                  PyObject *args)
 {
-    Py_ssize_t num_bytes, n;
+    Py_ssize_t num_bytes = -1, n;
     PyObject *result;
 
     CHECK_VALID(NULL);
-    if (!PyArg_ParseTuple(args, "n:read", &num_bytes))
+    if (!PyArg_ParseTuple(args, "|n:read", &num_bytes))
         return(NULL);
 
     /* silently 'adjust' out-of-range requests */