fittype() errors for an M-file function

I am trying to fit a data set using a custom M-file function. The function does well when called explicitly but gives me an error message when a part of a fittype() function. I am not sure what even can be wrong here. Here is the piece of the code:

yFit = Kin2UpDn(x,k1,k2,k3,k4,Btotal); % this works fine

BObs = fittype('Kin2UpDn(x,k1,k2,k3,k4,Btotal)'); % this is the next line; fails with the error message below

opt = fitoptions('Method','NonlinearLeastSquares','Algorithm','Levenberg-Marquardt','MaxIter',10,'Robust','Bisquare','Startpoint',[k1; k2; k3; k4; Btotal]); % this is the next line

Error: ??? Error using ==> fittype.fittype>fittype.fittype at 477 Expression Kin2UpDn(x,k1,k2,k3,k4,Btotal) is not a valid MATLAB expression, has non-scalar coefficients, or cannot be evaluated: Error in fittype expression ==> Kin2UpDn(x,k1,k2,k3,k4,Btotal) ??? Index exceeds matrix dimensions.

x is a vector; k1,k2,k3,k4,Btotal are scalars.

Many thanks for any help