Fixed in this changeset (http://hg.savannah.gnu.org/hgweb/octave/rev/a0875c6cab7e).  The error function now ignores empty struct inputs.

but now I get

octave:1> error(nargchk(1,5,4,'struct'))
error: invalid conversion of multi-dimensional struct to scalar struct

I still seem to get a non-empty struct:

octave:1> s=nargchk(1,5,4,'struct')
s =

  scalar structure containing the fields:

    message =
    identifier =

octave:2> whos s
Variables in the current scope:

   Attr Name        Size                     Bytes  Class
   ==== ====        ====                     =====  =====
        s           1x1                          0  struct

Total is 1 element using 0 bytes

octave:3> isempty(s)
ans = 0
octave:4> ver
----------------------------------------------------------------------
GNU Octave Version 3.5.0+
GNU Octave License: GNU General Public License
Operating System: Linux 2.6.38-11-generic #47-Ubuntu SMP Fri Jul 15 19:27:09 UTC 2011 x86_64
----------------------------------------------------------------------
no packages installed.


s should not be 1x1, but 1x0, 0x0 whatever... At least, isempty(s) should return "true".

nargchk behaviour differs between MATLAB and octave in cases like the following:

s = nargchk(1,5,4,'struct')

MATLAB returns

s =

0x1 struct array with fields:
    message
    identifier

Octave returns

s =

  scalar structure containing the fields:


octave:5> whos s
Variables in the current scope:

   Attr Name        Size                     Bytes  Class
   ==== ====        ====                     =====  =====
        s           1x1                          0  struct

I.e. MATLAB returns an empty struct with two fields, while Octave returns a 1-element struct with no fields. MATLAB has documented its nargchk behaviour as follows:

--- cited from MATLAB help
MSGSTRUCT = NARGCHK(LOW,HIGH,N,'struct') returns an appropriate message structure if N is not between LOW and HIGH. If N is in the specified range, the message structure is empty. The message structure has at a minimum two fields, 'message' and 'identifier'.
---

MATLAB code that relies on this behaviour will fail on Octave if N is in the specified range. This may be related to bug #33428.