diff --git a/src/mceditlib/nbtattr.py b/src/mceditlib/nbtattr.py
index 305e00f..dfe6a26 100644
--- a/src/mceditlib/nbtattr.py
+++ b/src/mceditlib/nbtattr.py
@@ -119,11 +119,16 @@ class NBTUUIDAttr(object):
         least = uuidInt & 0xffffffffffffffffL
         most = (uuidInt >> 64) & 0xffffffffffffffffL
         tag = instance.rootTag
-        tag["UUIDLeast"].value = least
-        tag["UUIDMost"].value = most
+        tag["UUIDLeast"].value = _signed(least)
+        tag["UUIDMost"].value = _signed(most)
         instance.dirty = True
 
 
+def _signed(bits):
+    if bits >= (1 << 63):
+        bits -= (1 << 64)
+    return bits
+
 class NBTCompoundRef(object):
     def __init__(self, rootTag, parent):
         """