Custom Proximity Search broken due to some change in custom search framework

The Proximity custom search now throws a fatal error as soon as you try to run it, triggered by this code in the constructor:

if ( ! isset( $this->_formValues['geo_code_1'] ) ||
! isset( $this->_formValues['geo_code_2'] ) ||
! isset( $this->_formValues['distance'] ) )

{ CRM_Core_Error::fatal( ts( 'Could not geocode input' ) ); }

I don't think any changes were mode to Proximity.php - so I'm guessing that we changed something in a base custom search class or ?? which is causing this. Not sure what other custom searches and / or reference code examples might be affected so posting as an issue.