diff --git a/habits.sh b/habits.sh
index f3acfc9..41c040b 100755
--- a/habits.sh
+++ b/habits.sh
@@ -1,6 +1,14 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
+if [ -z "$HABITS" ]; then
+  echo "Define a HABITS variable with a list of habits"
+fi
+
+if [ -z "$METRICS" ]; then
+  echo "Define a METRICS variable with a list of habits"
+fi
+
 HABITS_FILE="${HABITS_FILE:-$HOME/.local/share/os/habits.md}"
 
 usage() {