values of $\lambda$for $v(x,t):=u(\lambda x,6t)$ to be another solution of the heat equation?

Question

Let $Q=\mathbb{R}^n\times (0,\,+\infty)$ and $u:Q\to\mathbb{R}$ a solution (classical) of the heat equation $\partial_tu-D\Delta{u}=0 $in $Q$, where $D>0$ is an assigned coefficient. Let $v(x,t):=u(\lambda x,6t)$ with $ \lambda\in\mathbb{R}$. How many values of $\lambda$ are there for which the function $v$ is also a solution of the heat equation in $Q$?

My try:

$0=\partial_tv -D\Delta = \partial_t u(\lambda x,6t)-D\Delta u(\lambda x,6t) =6\partial_t u -D\lambda \Delta u$.

So I think I must choose $\lambda = 6$ as the only value.

However the correct answer should be that there are two values. How come? What I am doing wrong?