diff --git a/rplugin/python3/magma/outputchunks.py b/rplugin/python3/magma/outputchunks.py
index d2a27e0..2b9ce0e 100644
--- a/rplugin/python3/magma/outputchunks.py
+++ b/rplugin/python3/magma/outputchunks.py
@@ -122,7 +122,8 @@ def _get_char_pixelsize(self) -> Optional[Tuple[int, int]]:
             return None
 
         pty = get_pty_slave(os.getppid())
-        assert pty is not None
+        if pty is None:
+            return None
 
         with open(pty) as fd_pty:
             farg = struct.pack("HHHH", 0, 0, 0, 0)