PowerPC: `Error: Invalid operand` if register operands have letters

[SuperSamus]

For instance, this fails to assemble:

lis r0, 0x3c08
ori r0, r0, 0x8889
stw r0, -4(r1)
lfs f1, -4(r1)

In order to make it assemble, it needs to be changed to:

lis 0, 0x3c08
ori 0, 0, 0x8889
stw 0, -4(1)
lfs 1, -4(1)

This issue only happens when assembling. When disassembling, the Assembly is with the letters.
Which makes assembling the disassembled Assembly fail.