diff --git a/R/defaults.R b/R/defaults.R
index 45d2732dff..11c5f9b4b9 100644
--- a/R/defaults.R
+++ b/R/defaults.R
@@ -22,7 +22,10 @@ new_defaults = function(value = list()) {
   set2 = function(values) {
     old = get(names(values), drop = FALSE)
     if (length(values)) {
-      if (locked) stop('The object is read-only and cannot be modified.')
+      if (locked) stop(
+        'The object is read-only and cannot be modified. If you have to modify it ',
+        'for a legitimate reason, call the method $lock(FALSE) on the object before $set().'
+      )
       defaults <<- merge(values)
     }
     invisible(old)