set @start_value = @@wsrep_start_position;
set @@global.wsrep_start_position='foo:bar';
ERROR 42000: Variable 'wsrep_start_position' can't be set to the value of 'foo:bar'
set @@global.wsrep_start_position=NULL;
ERROR 42000: Variable 'wsrep_start_position' can't be set to the value of 'NULL'
SET @@global.wsrep_start_position = -1;
ERROR 42000: Incorrect argument type to variable 'wsrep_start_position'
set @@global.wsrep_start_position = @start_value;