isatty() should return a bool.
diff --git a/Objects/fileobject.c b/Objects/fileobject.c
index 3e0b85e..a752a20 100644
--- a/Objects/fileobject.c
+++ b/Objects/fileobject.c
@@ -565,7 +565,7 @@
 	Py_BEGIN_ALLOW_THREADS
 	res = isatty((int)fileno(f->f_fp));
 	Py_END_ALLOW_THREADS
-	return PyInt_FromLong(res);
+	return PyBool_FromLong(res);
 }