obscure error message for dot product of scalar functions

[campospinto]

When I write

u, v = elements_of(V0, names='u, v')
expr   = dot(u,v)

with a scalar-valued space V0 I get the following error message:

File "/Users/campos/Work/codes/sympde/sympde/calculus/core.py", line 302, in __new__
    a = reduce(mul, args_1)
TypeError: reduce() of empty sequence with no initial value

which is not very clear.
Either dot products should be defined for scalar valued functions (why not), or the message should be clearer