diff --git a/Core/memory.c b/Core/memory.c
index d3d9aaa6..148c9867 100644
--- a/Core/memory.c
+++ b/Core/memory.c
@@ -780,7 +780,7 @@ static void write_high_memory(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
                 }
                 else if ((gb->io_registers[GB_IO_JOYP] & 0x30) != (value & 0x30)) {
                     GB_sgb_write(gb, value);
-                    gb->io_registers[GB_IO_JOYP] = value & 0xF0;
+                    gb->io_registers[GB_IO_JOYP] = (value & 0xF0) | (gb->io_registers[GB_IO_JOYP] & 0x0F);
                     GB_update_joyp(gb);
                 }
                 return;