Why does this code compile properly, but make my Core flash red?

Troubleshooting
8

If I comment out all the lines of code and add them back in one at a time, this is the line that causes the hard fault:

SeeedRFID RFID(RFID_RX_PIN, RFID_TX_PIN);

So, it’s something in the library’s constructor that causes it to fail. Any ideas what that might be? It looks like others have used this library, but even so, maybe there’s a bug in the code for it?

And just to be certain, I’ve changed the RX & TX PIN to those on the Spark - 6 & 7