diff --git a/pymongo/_cbsonmodule.c b/pymongo/_cbsonmodule.c
index 9779c9b73..9d740af24 100644
--- a/pymongo/_cbsonmodule.c
+++ b/pymongo/_cbsonmodule.c
@@ -516,7 +516,7 @@ static PyObject* _elements_to_dict(PyObject* elements) {
             {
                 int length;
                 memcpy(&length, string + position, 4);
-                int subtype = (int)string[position + 4];
+                int subtype = (unsigned char)string[position + 4];
                 PyObject* data;
                 if (subtype == 2) {
                     data = PyString_FromStringAndSize(string + position + 9, length - 4);
diff --git a/test/test_bson.py b/test/test_bson.py
index 18fb2781b..103bf6405 100644
--- a/test/test_bson.py
+++ b/test/test_bson.py
@@ -114,6 +114,7 @@ class TestBSON(unittest.TestCase):
         helper({"an object": {"test": u"something"}})
         helper({"a binary": Binary("test", 100)})
         helper({"a binary": Binary("test", 128)})
+        helper({"a binary": Binary("test", 254)})
         helper({"another binary": Binary("test")})
         helper(SON([(u'test dst', datetime.datetime(1993, 4, 4, 2))]))