Question: Christoffel symbols for a metric with a vanishing diagonal term

Hello, I have a problem which might be very basic but I can't get over it.

I need to compute Christoffel symbols with the "Christoffel" command of the "Tensor" package. However, I noticed that whenever I give maple a metric with a vanishing diagonal element, I get the following error:

Error, (in DifferentialGeometry:-Tensor:-InverseMetric) expected 1st argument to be a metric tensor. Received: _DG([["tensor", M, [["cov_bas", "cov_bas"], []]], [`...`]])

For example, I tried with a very simple 2D case, giving coordinates (t,x) and a metric ds²=f(x)dtdx+h(x)dx²
for which I can compute by hand Christoffel symbols without any problem. The problem really seems to be about the missing diagonal element since if I add it, everything works fine.

By the way, I also noticed that if I apply the "InverseMetric" command I get exactly the same error, even though the matrix is invertible (except possibly at zeros of the functions f and h).

I will be grateful for any suggestion.