diff --git a/CHANGELOG.md b/CHANGELOG.md
index f271a3c98..0b8631032 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,7 @@ Find out more about isort's release policy [here](https://pycqa.github.io/isort/
   - Fixed #1779: Pylama integration ignores pylama specific isort config overrides.
   - Fixed #1781: `--from-first` CLI flag shouldn't take any arguments.
   - Fixed #1792: Sorting literals sometimes ignored when placed on first few lines of file.
+  - Fixed #1777: extend_skip is not honored wit a git submodule when skip_gitignore=true.
 
 ### 5.9.2 July 8th 2021
   - Improved behavior of `isort --check --atomic` against Cython files.
diff --git a/isort/settings.py b/isort/settings.py
index ee7297442..1acdb50d9 100644
--- a/isort/settings.py
+++ b/isort/settings.py
@@ -559,7 +559,7 @@ def _check_folder_gitignore(self, folder: str) -> Optional[Path]:
         git_options = ["-C", str(git_folder), "-c", "core.quotePath="]
         try:
             ignored = subprocess.check_output(  # nosec # skipcq: PYL-W1510
-                ["git", *git_options, "check-ignore", "-z", "--stdin"],
+                ["git", *git_options, "check-ignore", "-z", "--stdin", "--no-index"],
                 encoding="utf-8",
                 env=env,
                 input="\0".join(files),