Question: A problem with a result from solve

I have the following equation:

Diff(W(t), t) = -q*V*(sin(Phi)-sin(Psi[s]))/(2*h*Pi);

I solve it for rhs() = 0:

soln := solve([rhs((2)) = 0, Phi < 2*Pi], [Phi], allsolutions = true,explicit);

This works and I get this result:

Now I want to get the first zeros, which occur for _Z1 and _Z2 equal to 0. So I substitute:

subs(_Z2=0,(3));

and get

In other words, the substitution did not work.

The original problem is embedded in a larger sheet created with Maple 15 and there it does work. It fails on Maple 2015.2. I then pulled out the relevant pieces to make this example demonstrating the problem (see the attached sheet, which has some of my other (unsuccessful) attempts to diagnose what is going on). It seems like the created variables _Z1 and _Z2 are somehow not recognized at all.

The only way I can get the _Z2 terms out is to substitute 2=0. This is really too icky to seriously consider, though.

Anyone has seen this before?

FWIW: Maple 2015.2 on Mac OS X 10.10.5.

Thanks,

Mac Dude

Assumptions_test.mw