### non-negative integer relations on vertices

I would like to check whether there is a nonnegative integer relations between the vertices of my lattice polytope.
Here is my code (I copied from sage math document):

```
vertices = [(1,1,-1,-1,-1),(-1,-1,1,1,-1),(1,-1,-1,-1,1),(-1,1,1,1,1),(1,-1,1,-1,-1)]
p = LatticePolytope(vertices)
print p.vertices_pc()
print lattice_polytope.positive_integer_relations(p.vertices_pc().column_matrix())
```

Obviously, the sum of the first four vectors is zero vector. However, sage returns an error:

TypeError: unable to make sense of Maxima expression '"Problemnotfeasible!"' in Sage

I would like to understand the reason. Is there some obvious thing I am missing? Interestingly, if I remove the last vertex from the list, then it gives the expected relation.