diff --git a/src/define.sh b/src/define.sh
index 89ce2cc..f6be22b 100644
--- a/src/define.sh
+++ b/src/define.sh
@@ -1,6 +1,7 @@
 #!/usr/bin/env bash
 set -Eeuo pipefail
 
+: "${KEY:=""}"
 : "${WIDTH:=""}"
 : "${HEIGHT:=""}"
 : "${VERIFY:=""}"