Here's the definition of the at macro from http://repo.or.cz/nasm.git/blob/HEAD:/macros/standard.mac#l109

===
%imacro at 1-2+.nolist
    times (%1-%$strucname)-($-%$strucstart) db 0
    %2
%endmacro
===

Here's my test case:

===
$ nasm -v
NASM version 2.13.02
$ cat test.asm 
	struc EXEHEADER
exeHeaderSizePara:	resw 1
	endstruc

	istruc EXEHEADER
at exeHeaderSizePara,
	iend
$ nasm test.asm 
test.asm:6: error: expecting `)'
$ nasm test.asm -E
%line 1+1 test.asm
[absolute 0]
%line 1+0 test.asm
EXEHEADER:
%line 2+1 test.asm
exeHeaderSizePara: resw 1
EXEHEADER_size equ ($-EXEHEADER)
%line 3+0 test.asm
[section .text]
%line 4+1 test.asm

..@4.strucstart:
times (exeHeaderSizePara,-EXEHEADER)-($-..@4.strucstart) db 0
%line 6+0 test.asm

%line 7+1 test.asm
times EXEHEADER_size-($-..@4.strucstart) db 0
$ 
===

As is visible in the only-preprocessed output, the trailing comma is taken as belonging to the first macro parameter, resulting in that parameter's expansion being "exeHeaderSizePara,". I expected it to signify an empty second parameter, and the comma itself being parsed as the parameter separator.