diff --git a/cpu/decoder.cpp b/cpu/decoder.cpp
index d7c2c0b..a208c5b 100644
--- a/cpu/decoder.cpp
+++ b/cpu/decoder.cpp
@@ -192,7 +192,7 @@ void Cpu::executeInstruction()
       switch(prefix_op & 0xC0)
         {
         case 0x00:
-          switch(prefix_op & 0xF1)
+          switch(prefix_op & 0xF8)
             {
             case 0x00: // RLC
               {