C-x C-f in two frames -> "user minibuffer while in minibuffer": Bug?

[David Reitter]

I don't know if this is a bug or not, so I'll post it here:

If I open a second frame, then do C-x C-f in one of them and press  
tab so that the window is split and I get a *Completions* buffer in  
one frame, and when I then select the second frame and do a C-x C-f  
there, I don't get another *Completions* buffer there, but an error  
message that appears in the first frame:

"Command attempted to use minibuffer while in minibuffer"

This happens with standard settings, on the GNU/Linux port as well as  
on Carbon, no matter what display-buffer-reuse-frames is.

Is that a bug?

D