Matlab Array indices must be positive integers or logical values

I've got the following problem: Every time I run the code below, I get the message: "Array indices must be positive integers or logical values." Each variable is a column vectore (size 300x1 each). I'm not able to figure out where the bug could be since the index variables are never negative (second and third if-clause).

Could anybody help me out? Thank you so much!

Amount = zeros(size(Yield));
Current = zeros(size(Yield));
Current(1,1) = 2000;  

M = [Prob1,Prob2,Prob3];
ProbMax = max(M,[],2);
N = [Prob1,Prob2,Prob3];
ProbMin = min(N,[],2);
P = [Probyie1,Probyie2,Probyie3];
ProbyieMax = min(P,[],2);


for i=1:length(Yield)

    if      ismember(ProbMax(i),Prob1(i)) & ... 
            ProbMax(i)<0.9 & ProbMin(i)>0 & ...
            (0.45 <= ProbMax(i)) & (ProbMax(i) <= 0.75) & ...  
            Current(idx)>0 & ... 
            ProbMax>ProbyieMax 
                idx = floor((i-1)/9)*9+1;       
                if mod(i,9)==1 
                    Current(idx) = Amount(idx-9:idx-1) + Current(idx-9);
                end
                Amount(i)=(Current(idx)*0.1*(Prob1(i)-((1-Prob1(i))/(Yield(i)-1)))); 
    else  
                idx = floor((i-1)/9)*9+1;
                if mod(i,9)==1
                    Current(idx) = Amount(idx-9:idx-1) + Current(idx-9);
                end
                Amount(i)=0;
    end
end