diff --git a/extensions/vp9/README.md b/extensions/vp9/README.md
index 941b413c09..649e4a6ee2 100644
--- a/extensions/vp9/README.md
+++ b/extensions/vp9/README.md
@@ -28,7 +28,8 @@ EXOPLAYER_ROOT="$(pwd)"
 VP9_EXT_PATH="${EXOPLAYER_ROOT}/extensions/vp9/src/main"
 ```
 
-* Download the [Android NDK][] and set its location in an environment variable:
+* Download the [Android NDK][] and set its location in an environment variable.
+Only versions up to NDK 15c are supported currently (see [#3520][]).
 
 ```
 NDK_PATH="<path to Android NDK>"
@@ -70,6 +71,7 @@ ${NDK_PATH}/ndk-build APP_ABI=all -j4
 
 [top level README]: https://github.com/google/ExoPlayer/blob/release-v2/README.md
 [Android NDK]: https://developer.android.com/tools/sdk/ndk/index.html
+[#3520]: https://github.com/google/ExoPlayer/issues/3520
 
 ## Notes ##