Hi AFNI'ers

I'm trying to render a functional dataset on a structural from the same study using the Render[new] plugin. The "Choose Underlay dataset..." option in the plugin window does not include the structural in the list even though it exists in the same directory (it is also the current underlay in the main window). Why won't the plugin recognize this as a valid underlay dataset? Must I change the datum type? Thanks for your help.
Heres the 3dinfo on it:


Dataset File: nksSub01+tlrc
Identifier Code: XYZ_Z1MECuI87bxKhhKd-D1vyw Creation Date: Thu Jun 2 14:38:39 2005
Dataset Type: Spoiled GRASS (-spgr)
Byte Order: MSB_FIRST [this CPU native = MSB_FIRST]
Storage Mode: BRIK file
Data Axes Orientation:
first (x) = Right-to-Left
second (y) = Posterior-to-Anterior
third (z) = Inferior-to-Superior [-orient RPI]
R-to-L extent: -90.000 [R] -to- 91.000 [L] -step- 1.000 mm [182 voxels]
A-to-P extent: -91.000 [A] -to- 126.000 [P] -step- 1.000 mm [218 voxels]
I-to-S extent: -72.000 -to- 109.000 [S] -step- 1.000 mm [182 voxels]
Number of values stored at each pixel = 1
-- At sub-brick #0 '#0' datum type is float: 0 to 171.958