0221BASE45 -tp FREESURFERRECON_TEST_005_S_0221_45_20070321.105506_1 -tp FREESURFERRECON_TEST_005_S_0221v2_45_20061006.104318_1 -all -force

%% I get this error:
%%%%%%%%%%
mri_concatenate_lta -invert1 FREESURFERRECON_TEST_005_S_0221_45_20070321.105506_1_to_FREESURFERRECON_TEST_005_S_0221BASE45.lta identity.nofile FREESURFERRECON_TEST_005_S_0221BASE45_to_FREESURFERRECON_TEST_005_S_0221_45_20070321.105506_1.lta

invert the first LTA before applying it
MatrixInverse: NULL input matrix!

Read individual LTAs
%%%%%%%%%%

Any ideas on how to solve this?

A side note for other troubleshooters: getting to this stage takes around 5 minutes with 2 subjects and around 90 minutes with 3.

thanks!
Matt