diff --git a/git/diff.py b/git/diff.py
index aba1a108..95f17c97 100644
--- a/git/diff.py
+++ b/git/diff.py
@@ -163,6 +163,12 @@ class Diffable:
         # Fixes: https://github.com/gitpython-developers/GitPython/issues/172
         args.append("--no-color")
 
+        # When diff.external is configured, git may try to use the external
+        # diff tool instead of producing diff output. Add --no-ext-diff to
+        # ensure git produces diff output regardless of external diff settings.
+        # Fixes: https://github.com/gitpython-developers/GitPython/issues/852
+        args.append("--no-ext-diff")
+
         if paths is not None and not isinstance(paths, (tuple, list)):
             paths = [paths]