A minus sign on the truncation error: Crank-Nicolson method for a diffusion equation with consume

Question

I was studying by first time truncation error on finite schemes and the author of the article I am studying states the equation:

$u_t=u_{xx}-1$

and the truncation error

$T_m^{n+1}=\dfrac{\partial u}{\partial t}(x_m,t_{n+1/2})-\dfrac{\partial^2 u}{\partial x^2}(x_m,t_{n+1/2})\fbox{$-1$}$.

I was wondering why the minus sign, i.e., I thought by intuition it would be:

$T_m^{n+1}=\dfrac{\partial u}{\partial t}(x_m,t_{n+1/2})-\dfrac{\partial^2 u}{\partial x^2}(x_m,t_{n+1/2})\fbox{+1}$.

Thank you in advance to explain because this is non-intuitive.

Answer 1

I contacted the author and he told me there's a typo here and the signal must be $+$. Many thanks for attention.