diff --git a/website/src/models/user.js b/website/src/models/user.js
index c77df6ab56..7000cdf5a8 100644
--- a/website/src/models/user.js
+++ b/website/src/models/user.js
@@ -539,6 +539,9 @@ UserSchema.pre('save', function(next) {
 
 UserSchema.methods.unlink = function(options, cb) {
   var cid = options.cid, keep = options.keep, tid = options.tid;
+  if (!cid) {
+    return cb("Could not remove challenge tasks. Please delete them manually.");
+  }
   var self = this;
   switch (keep) {
     case 'keep':