Can index solution by functions of parameters, but not pure parameters

[hersle]

The last line in

using ModelingToolkit, OrdinaryDiffEq, Test
using ModelingToolkit: t_nounits as t, D_nounits as D
vars = @variables x(t)
pars = @parameters x0
@named S = ODESystem([D(x) ~ 0.0], t, vars, pars; defaults = [x => x0])
S = structural_simplify(S)
prob = ODEProblem(S, [], (0.0, 1.0), [x0 => 1.0])
sol = solve(prob)
sol[x][begin] # works
sol[x0*2] # works
sol[x0] # fails

fails with

ERROR: Indexing with parameters is deprecated. Use `getp(A, x0)` for parameter indexing.

I think that if indexing by x0*2 works, then so should x0.