diff --git a/bson/_cbsonmodule.c b/bson/_cbsonmodule.c
index bdd987e53..d912524f4 100644
--- a/bson/_cbsonmodule.c
+++ b/bson/_cbsonmodule.c
@@ -1040,9 +1040,11 @@ static PyObject* _cbson_dict_to_bson(PyObject* self, PyObject* args) {
     PyObject* result;
     unsigned char check_keys;
     unsigned char uuid_subtype;
+    unsigned char top_level = 1;
     buffer_t buffer;
 
-    if (!PyArg_ParseTuple(args, "Obb", &dict, &check_keys, &uuid_subtype)) {
+    if (!PyArg_ParseTuple(args, "Obb|b", &dict,
+                          &check_keys, &uuid_subtype, &top_level)) {
         return NULL;
     }
 
@@ -1052,7 +1054,7 @@ static PyObject* _cbson_dict_to_bson(PyObject* self, PyObject* args) {
         return NULL;
     }
 
-    if (!write_dict(self, buffer, dict, check_keys, uuid_subtype, 1)) {
+    if (!write_dict(self, buffer, dict, check_keys, uuid_subtype, top_level)) {
         buffer_free(buffer);
         return NULL;
     }