Fix compilation error in debug mode.
diff --git a/Modules/_io/stringio.c b/Modules/_io/stringio.c
index 0fc8078..a5bfd79 100644
--- a/Modules/_io/stringio.c
+++ b/Modules/_io/stringio.c
@@ -583,9 +583,6 @@
     Py_CLEAR(self->writenl);
     Py_CLEAR(self->decoder);
 
-    assert((newline != NULL && newline_obj != Py_None) ||
-           (newline == NULL && newline_obj == Py_None));
-
     if (newline) {
         self->readnl = PyString_FromString(newline);
         if (self->readnl == NULL)