Cannot save the settings page options. A error is showing.

While I am pressing the Save Settings button a fatal error is showing top of the page and the options I set is not saving. The error message is like follwoing
“cannot unset sting offsets in rootpath/wp-includes/capabilities.php on line 233”.

and The capabilities.php on line 233 contains like following
229. function remove_cap( $role, $cap ) {
230. if ( ! isset( $this->roles[$role] ) )
231. return;
232.
233. unset( $this->roles[$role][‘capabilities’][$cap] );
234. if ( $this->use_db )
235. update_option( $this->role_key, $this->roles );
236. }

http://wordpress.org/plugins/wpmarketplace/