diff --git a/@planetarium/lib9c/CONTRIBUTING.md b/@planetarium/lib9c/CONTRIBUTING.md
index e680cec222..a071c56a5f 100644
--- a/@planetarium/lib9c/CONTRIBUTING.md
+++ b/@planetarium/lib9c/CONTRIBUTING.md
@@ -22,6 +22,13 @@ pnpm build
 
 `@planetarium/lib9c` uses `Lib9c.Tools action analyze` command to check whether implemented actions make valid bencodex value. You should build the .NET project first. **If the .NET Lib9c project is changed, you must build `Lib9c.Tools` project again.**
 
+if you haven't pulled submodules, run this command first.
+
+```
+git submodule update --init --recursive
+```
+after
+
 ```
 dotnet build ../../.Lib9c.Tools/Lib9c.Tools.csproj
 ```