[FieldTrip] ft_sourceinterpolate after grandaverage, interpn error

[cornelius abel]

Dear Fieldtrip list,

i successfully beamformed some steady state activity and the sourceplots 
per subject are looking very nice.
As a next step I tried to calculate a grand average using 
ft_sourcegrandaverage:

cfg=[];
cfg.keepindividual='no';
cfg.parameter='avg.pow';
GACont= ft_sourcegrandaverage(cfg, AllsourceCont{:});

giving me the following structure:

GACont =
pos: [3570x3 double]
dim: [17 14 15]
avg: [1x1 struct]
var: [1x1 struct]
dimord: 'voxel'
inside: [1846x1 double]
outside: [1724x1 double]
df: [3570x1 double]
cfg: [1x1 struct]

but interpolating this onto an anatomy always gives me an error:

cfg = [];
cfg.downsample= 2;
cfg.parameter = 'avg.avg.pow';
sourceDiffInt= ft_sourceinterpolate(cfg, GACont , mri);

/??? Error using ==> interpn at 155/
///Wrong number of input arguments or some dimension of V is less than 2./
///Error in ==> ft_sourceinterpolate>my_interpn at 464/
///av(sel) = interpn(fv, ax(sel), ay(sel), az(sel), interpmethod);/
///Error in ==> ft_sourceinterpolate at 368/
///av( sel) = my_interpn(fv, ax(sel), ay(sel), az(sel), 
cfg.interpmethod, cfg.feedback);/

however interpolating a single subject source data works just fine...

cfg = [];
cfg.downsample= 2;
cfg.parameter = 'avg.pow';
sourceDiffInt= ft_sourceinterpolate(cfg, AllsourceCont{1} , mri);


What am i doing wrong here, any ideas?



Greetings, Cornelius