The author proves by very standard methods the stability in the sense of Ulam of the functional equation \[ Q(a_1x_1+a_2x_2)+Q(a_2x_1-a_1x_2)=(a_1^2+a_2^2)[Q(x_1)+Q(x_2)] \] where \(Q:X \to Y\), \(X, Y\) are normed linear space and \(Y\) is complete, and \((a_1,a_2) \in \mathbb R^2 \setminus (0,0)\) is arbitrarily fixed.