Simulink Modelling: Parameter estimation:Error - invalid coverage path

Can someone explain to me what is the meaning of invalid coverage path for a subsystem? The following lines are shown in the error:

1) if isempty(rootSlHandle) warning('SLVNV:simcoverage:cv_init_dialog_test:InvalidPath','Invalid coverage path, ''%s''',coveragePath); rootSlHandle = modelH; coveragePath = ''; end

The string in bold letters are shown in the warning. Please help, its urgent.