Can I change the output of the ode45 function?

I want to use ode45 to solve a 1st order differential equation but my input parameter is a matrix, not a column vector. I get an error message that says:

" Error using odearguments (line 90) VONNEUMANN must return a column vector.

Error in ode45 (line 113) [neq, tspan, ntspan, next, t0, tfinal, tdir, y0, f0, odeArgs, odeFcn, ...

Error in First_Density_Matrix_Model (line 31) [T,Y] = ode45(@vonNeumann, tspan, y0); "

where VONNEUMANN is the differential equation I want to solve and First_Density_Matrix_Model is the programme it is being called in.

Is there any way I can change the output parameters to allow me to do this?

Thanks!