Button color not inverting on mouseover

Site URL: http://www.letterand.ink

Hi all, just discovered that the button in my header isn't behaving correctly on rollover. The rest of the buttons on my site are fine (the background becomes solid and the text becomes white on rollover) but this one isn't. Any thoughts?

Img 1: Button / Img 2: Button on rollover (text is not visible)

You rock, thanks so much for your help!