Error: Assignment has more non-singleton rhs dimensions than non-singleton subscripts

I want to fit the following model to a set of data. The input data is P. A 5x243 Matrix. The output data is B. A 3x243 Matrix. MatLab is giving me the following Error:

>> [estimates, model] = fitcurvedemo(P, B)
Assignment has more non-singleton rhs dimensions than non-singleton subscripts

Error in fminsearch (line 189)
fv(:,1) = funfcn(x,varargin{:});

Error in fitcurvedemo (line 4)
estimates = fminsearch(model, start_point);

I have absolutely no idea why something is wrong with the dimensions. I have checked them several times but can't find a mistake. I have the feeling it could be because of the parameters. It would be great, if someone could help me! Thanks in advance! Below is my code:

function [estimates, model] = fitcurvedemo(P, B)
start_point = rand(5, 1);
model = @model3n;
estimates = fminsearch(model, start_point);
    function [sse, FittedB] = model3n(params)
rho = params(1);   
a = params(2);
b = params(3);
c = params(4);
d = params(5);
V = a.*sqrt(2/rho.*abs(P));

for k = 1:243
    if P(1,k)>=0
       Z(1,k) = -V(1,k);
    else Z(1,k) = V(1,k);
    end
end

for k = 1:243
    if P(2,k)>=0
       Z(2,k) = -V(2,k);
    else Z(2,k) = V(2,k);
    end
end

   for k = 1:243
      if P(3,k)>=0
         Z(3,k) = V(3,k);
      else Z(3,k) = -V(3,k);
      end
   end

   for k = 1:243
      if P(4,k)>=0
         Z(4,k) = -V(4,k);
      else Z(4,k) = V(4,k);
      end
   end

   for k = 1:243
      if P(5,k)>=0
         Z(5,k) = V(5,k);
      else Z(5,k) = -V(5,k);
      end
   end

      alpha = b.*Z(1,:);
      betha =c.*(Z(2,:)-Z(3,:));
      velocity =d.*(Z(4,:)-Z(5,:));

      FittedB = [alpha;betha;velocity];

          ErrorVector = FittedB - B;
          sse = sum(ErrorVector .^ 2);
      end
  end

[Pawel Ladosz]

what is the model3n function?