diff --git a/adafruit_trellis.py b/adafruit_trellis.py
index f2a2180..185f849 100644
--- a/adafruit_trellis.py
+++ b/adafruit_trellis.py
@@ -91,7 +91,7 @@ def __setitem__(self, x, value):
         led = ledLUT[x % 16] >> 4
         mask = 1 << (ledLUT[x % 16] & 0x0f)
         if value:
-            self._parent._led_buffer[x // 16][(led * 2) + 1] |= mask
+            self._parent._led_buffer[x // 16][(led * 2) + 1] |= (mask & 0xff)
             self._parent._led_buffer[x // 16][(led * 2) + 2] |= mask >> 8
         elif not value:
             self._parent._led_buffer[x // 16][(led * 2) + 1] &= ~mask