subscript out of range, can't see why

I get <subscript out of range> as a tooltip when debugging this (bold) code. It's probably obvious, but I can't see why.

Test() As String and H% are both listed with other (working) variables as 'Public' in another module, including Z_max, Z_Inside, Tmp1.

For H = 0 To Z_max
Test(H) = "Z " & Z_Inside
Tmp1 = Tmp1 + Test(H)
Z_Inside = Z_Inside +2
Next H

I didn't think I missed anything out, but I must have done.