diff --git a/Library/Homebrew/cmd/casks.sh b/Library/Homebrew/cmd/casks.sh
index 4a8d866f45..aa3a75cf2b 100644
--- a/Library/Homebrew/cmd/casks.sh
+++ b/Library/Homebrew/cmd/casks.sh
@@ -3,7 +3,9 @@
 #:  List all locally installable casks including short names.
 #:
 
-source "$HOMEBREW_LIBRARY/Homebrew/items.sh"
+# HOMEBREW_LIBRARY is set in bin/brew
+# shellcheck disable=SC2154
+source "${HOMEBREW_LIBRARY}/Homebrew/items.sh"
 
 homebrew-casks() {
   homebrew-items 'Formula' 's|/Casks/|/|' '^homebrew/cask'