diff --git a/Core/apu.c b/Core/apu.c
index 8935533..ec4b1d4 100755
--- a/Core/apu.c
+++ b/Core/apu.c
@@ -491,6 +491,9 @@ void GB_apu_write(GB_gameboy_t *gb, uint8_t reg, uint8_t value)
         case GB_IO_NR21: {
             unsigned index = reg == GB_IO_NR21? GB_SQUARE_2: GB_SQUARE_1;
             gb->apu.square_channels[index].pulse_length = (0x40 - (value & 0x3f));
+            if (!gb->apu.global_enable) {
+                gb->io_registers[reg] &= 0x3f;
+            }
             break;
         }