Dependent Substitute DropList not working

DropList in B4 is populated by Main List in B12 - B16, and works perfectly.

Dependent DropList in C4 should be populated by "Daughter" ranges in cols C - G, but currently defaults to an Error.

DV formula for C4 is =INDIRECT(SUBSTITUTE($B$4,"_",""))

Any pointers or solutions as to why this isn't working are welcome as ever,

Ochimus