Keep getting the error "Index exceeds the number of array elements (1)"

Can someone please explain why I keep getting this error? Here is my code:

for Count = 1:100
     fprintf('Count = %d\n', Count');
end 
 
Wait = input('Press return to continue', 's');
 
x = 1;  
while x <= 100        % test Count
     disp(x)
     x = x + 1;  % modify Count
end
 
disp('Prime Numbers')
for num = 1:100
    i = isprime(num);
    primenum = num(i);
    disp(primenum);
end 

It is showing me that the error is located at the "primenum = num(i);" I am not sure why.