Ronjon's is the way to go. The block names there are in list form rather than trying to filter them all out with '((2 . "aaa,bbb,ccc,ddd,eee,...etc.))
Cadmoogle, are you sure your way actually worked with a shorter list?
Try checking the selection set length with (sslength blk) after running and see if you get the correct results.
Here is what I did [with 6 inserts of "testblk1" and 4 inserts of "testblk2"]:
(sslength(setq blk(ssget "x" '((0 . "INSERT")(2 .
"testblk1,testblk2"))))) ;;;returns 10
(sslength(setq blk(ssget "x" '((0 . "INSERT")(2 .
"testblk1,
testblk2")))));;;returns 6...it only counts the first block...maybe because it is looking for " testblk2" or "\ntestblk2"...neither of which exist
Either way, you don't get an error, but the selection set is not what you were expecting.
Or maybe it doesn't even check for those. I'm not sure if those are valid block names. Very interesting though.
Might explain that error you were getting though. Not sure.