Undefined variable "m" after setting the "from email address"

The error appear after setting the "From email address" in the "Protected node email support" fieldset at admin/config/protected_node

Notice: Undefined variable: m in _protected_node_admin_settings_validate() (line 553 of .../sites/all/modules/protected_node/protected_node.settings.inc).

Only one line need to be changed, patch following.