diff --git a/runtime/syntax/sil.vim b/runtime/syntax/sil.vim
index 5583497..43158da 100644
--- a/runtime/syntax/sil.vim
+++ b/runtime/syntax/sil.vim
@@ -36,6 +36,7 @@
       \ c
       \ method
       \ objc_method
+      \ sil_differentiability_witness
       \ thick
       \ thin
       \ witness_method