[Gimp-user] why no difference of gaussians on layer masks? gimp 2.7.5

[Gary Aitken <garya dreamchaser org>]

Hello all,

gimp 2.7.5 on winXP, unfortunately.  New hardware problems on my freebsd 
system.  grr.  Anyway...

Why won't the gimp allow me to use a difference of gaussians on a layer 
mask?  This seems like a perfectly fine thing to do for edge detection, 
and indeed, other edge-detect algorithms work:

For example:
  New layer mask, from grayscale copy of layer
  Filters / edge-detect / difference of gaussians says:
    Execution error for 'Difference of Gaussians':
    Can operate on layers only (but was called on channel or mask).
but
  Filters / edge-detect / Edge...
    works fine

Is this a bug, or am I missing something?
Any insights would be appreciated,

Gary