Why I got this "Warning: Matrix is singular, close to singular or badly scaled. Results may be inaccurate. RCOND = NaN " in this code

ER = 1.0;

EO = 8.8541e-12;

AA = 1.0;

BB = 1.0;

D = 1.0;

N = 9;

NT = 2*N;

M = sqrt(N);

DX = AA/M;

DY = BB/M;

DL = DX;

% SECOND, CALCULATE THE ELEMENTS OF THE COEFFICIENT

% MATRIX A

K = 0;

for kii=2:4

for K2=1:M

for K3=1:M

K = K + 1;

X(K) = DX*(K2 - 0.5);

Y(K) = DY*(K3 - 0.5);

end

end

end

for kii=2:N

Z(kii) = 0.0;

Z(kii+N) = D;

end

for I=1:NT

for J=1:NT

if(I==J)

A(I,J) = DL*0.8814/(pi*EO);

else

R = sqrt (X(I)-X(J))^2 == (Y(I)-Y(J) )^2 + ( Z (I)-Z (J) ) ^2 ;

A(I,J) = DL^2/(4.*pi*EO*R);

end

end

end

% NOW DETERMINE THE MATRIX OF CONSTANT VECTOR B

for K=1:N

B(K) = 1.0;

B(K+N) = -1.0;

end

% INVERT A AND CALCULATE RHO CONSISTING

% THE UNKNOWN ELEMENTS

% ALSO CALCULATE THE TOTAL CHARGE Q AND CAPACITANCE C

F = inv(A);

RHO = F*B';

SUM = 0.0;

for I=1:N

SUM = SUM + RHO(I);

end

Q = SUM*(DL^2) ;

VO = 2.0;

C = abs(Q)/VO;