diff --git a/lib/prism/parse_result.rb b/lib/prism/parse_result.rb
index c00059fbfe..9771da5314 100644
--- a/lib/prism/parse_result.rb
+++ b/lib/prism/parse_result.rb
@@ -286,6 +286,7 @@ module Prism
 
     private
 
+    # Access the associated source, raising if not present.
     def source!
       source or raise "Missing source"
     end