diff --git a/Build.md b/Build.md
index f363e2e..2eee9c8 100644
--- a/Build.md
+++ b/Build.md
@@ -32,6 +32,7 @@ cmake --build Release
 ```
 
 The resulting executable will be `Release/ccls`.
+If you forgot to pass `--recursive` when cloning the repo, `git submodule update --init` to clone the rapidjson repository.
 
 If you compile ccls with GCC 8.2.1, append `-DCMAKE_CXX_FLAGS=-fno-gnu-unique`. See https://github.com/MaskRay/ccls/issues/363#issuecomment-482625854 for details.