"only g-code for fdm printer can be uploaded"  

"only g-code for fdm printer can be uploaded"

I have a Prusa MK3S with an attached Raspberry Pi Zero 2 W running PrusaLink. In Prusaslicer, I can slice a model but when I click the Send to Printer button, PrusaSlicer fives this error:

HTTP 415: Unsupported Media Type
Only G-Code for FDM printer can be uploaded.

I'm using the Original Prusa i3 MK3S & MK3S+ preset, so it's a mystery to me why it thinks it's not GCode for an FDM printer. In fact, if I export the Gcode to a file, upload it in the PrusaLink browser page, it uploads and prints just fine. 
I've attached the offending gcode but it happens with every model I try to slice and print. I'm using PrusaSlicer 2.9.4 on Ubuntu. Here's the PrusaLink version info:

API 2.0.0
Firmware 3.14.1-8237
Server 0.8.1
SDK 0.8.1
Frontend 3.12.0

Any idea what could be happening?