diff --git a/libexec/rbenv-whence b/libexec/rbenv-whence
index 30613eed..b6809a56 100755
--- a/libexec/rbenv-whence
+++ b/libexec/rbenv-whence
@@ -18,5 +18,10 @@ whence() {
 }
 
 RBENV_COMMAND="$1"
+if [ -z "$RBENV_COMMAND" ]; then
+  echo "usage: rbenv whence [--path] COMMAND" >&2
+  exit 1
+fi
+
 result="$(whence "$RBENV_COMMAND")"
 [ -n "$result" ] && echo "$result"