BT NQueens Matlab does not return answer (array)

i want the first array answer, but can't return that. if i change last line to BT(array,1,n), code does work correctly, but without returning anything.

error is:

Output argument "answer" (and maybe others) not assigned during call to "BT".

Error in Run (line 5)

a = BT(array,1,n)

main:

clc
clear
array = [];
n = input('chess board is ?*?, inter ?: ');
a = BT(array,1,n) 

my BT funcntion:

function answer = BT(array,index,n)
    if (is_valid(array,index))
        if (index==n+1)
            disp(array)
            answer = array;
            return            
        end
        for i=1:n
           array(index)= i;
           BT(array,index+1,n);   
        end      
    end
end

is_valid (works correctly):

function [flag] = is_valid(array,index)
    for i=1:index-1
        for j=i+1:index-1
            if (array(j) == array(i)) || (abs(array(i)-array(j))==abs(i-j))
               flag = false;
               return
            end
        end
    end
    flag = true;
     
end