diff --git a/certbotd.sh b/certbotd.sh
index 7989c12..db01da0 100755
--- a/certbotd.sh
+++ b/certbotd.sh
@@ -50,7 +50,12 @@ case $1 in
   # Generate certificates
   *)
     # Only one instance can run at a time
-    test -f "${lock}" && exit 0
+    if test -f "${lock}" ; then
+      echo "There's a certbotd instance already running, doing nothing..." >&2
+      echo "If the problem persists, you may need to remove ${lock} manually." >&2
+      exit 1
+    fi
+
     touch "${lock}"
 
     # Save headers here