"em" or "iem" are the endings for both masc. and neut. Instr.
you use "em" except when the stem ends in a velar (g or k), when you use "iem". There's a spelling rule in Polish that you should try and avoid "e" after velars...
"The letter e is usually separated from a preceding k or g by i,"
Swan, Oscar (2008-10-12). Polish Verbs & Essentials of Grammar, Second Edition (Verbs and Essentials of Grammar Series) (Kindle Location 201). McGraw-Hill Education. Kindle Edition.
The notes about the instrumental case back in those exercise were a bit sketchy a while back but they've tightened them up a bit since...
According to this wikipedia article, it is an irregular noun (scroll down passed 'Neuter Nouns'), but possibly only in the plural sense. As far as I can tell, it doesn't explicitly tell what gender the word is, or how it declines. I think it may be because of hard/soft sounds, but I am not entirely sure. Article: https://en.m.wikipedia.org/wiki/Polish_morphology#Nouns