Why am I getting the error "Array indices must be positive integers or logical values."?

Question

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function [Fluindex ] = find_Findex(Signal,timeframe,SampRate,slide )
%FIND_Kurtosis Summary of this function goes here
%   Detailed explanation goes here
channels=size(Signal,2);
nFrames=size(Signal,1)/(timeframe*SampRate);
Fluindex=[];
if slide <=0 
    partition=1;
else
    partition=slide/timeframe;
end
for f = 1:partition:nFrames
    start_ind  =((f-1)*timeframe*SampRate)+1;
    end_ind  = f*timeframe*SampRate;
    Fluind_frame=[];
    for i= 1:channels
        Findex = zeros(2,3,4);
        size(Findex)
        Fluind_frame = zeros(3,4,5,6);
        size( Fluind_frame)
        Fluind_frame(:,i)= Findex (Signal(start_ind:end_ind,i));
    end
    Fluindex=cat(1,Fluindex,Fluind_frame);
end
end
when i run above code its show error like this.
Error using find_Findex 
Array indices must be positive integers or logical values.
"why its showing like this??"