diff --git a/src/crab/gba/thumb/move_shifted_register.cr b/src/crab/gba/thumb/move_shifted_register.cr
index f21d439..13637c0 100644
--- a/src/crab/gba/thumb/move_shifted_register.cr
+++ b/src/crab/gba/thumb/move_shifted_register.cr
@@ -10,7 +10,8 @@ module GBA
       when 0b00 then set_reg(rd, lsl(@r[rs], offset, pointerof(carry_out)))
       when 0b01 then set_reg(rd, lsr(@r[rs], offset, true, pointerof(carry_out)))
       when 0b10 then set_reg(rd, asr(@r[rs], offset, true, pointerof(carry_out)))
-      else           raise "Invalid shifted register op: #{op}"
+      when 0b11 # encodes thumb add/subtract
+      else raise "Invalid shifted register op: #{op}"
       end
       set_neg_and_zero_flags(@r[rd])
       @cpsr.carry = carry_out