diff --git a/src/jupytext/contentsmanager.py b/src/jupytext/contentsmanager.py
index 812e7dac..dd6a9564 100644
--- a/src/jupytext/contentsmanager.py
+++ b/src/jupytext/contentsmanager.py
@@ -299,7 +299,11 @@ def read_one_file(alt_path, alt_fmt):
 
                 self.log.info(f"Reading SOURCE from {alt_path}")
                 text = self.super.get(
-                    alt_path, content=True, type="file", format="text"
+                    alt_path,
+                    content=True,
+                    type="file",
+                    # Don't use the parent format, see https://github.com/mwouts/jupytext/issues/1124
+                    format=None,
                 )["content"]
                 return reads(text, fmt=alt_fmt, config=config)