diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -4249,13 +4249,19 @@ group id.");
 static PyObject *
 posix_initgroups(PyObject *self, PyObject *args)
 {
+    PyObject *oname;
     char *username;
+    int res;
     long gid;
 
-    if (!PyArg_ParseTuple(args, "sl:initgroups", &username, &gid))
-        return NULL;
-
-    if (initgroups(username, (gid_t) gid) == -1)
+    if (!PyArg_ParseTuple(args, "O&l:initgroups",
+                          PyUnicode_FSConverter, &oname, &gid))
+        return NULL;
+    username = PyBytes_AS_STRING(oname);
+
+    res = initgroups(username, (gid_t) gid);
+    Py_DECREF(oname);
+    if (res == -1)
         return PyErr_SetFromErrno(PyExc_OSError);
 
     Py_INCREF(Py_None);