diff --git a/zc_utils.py b/zc_utils.py
index 13e9fa9..1fa2c58 100755
--- a/zc_utils.py
+++ b/zc_utils.py
@@ -3,7 +3,10 @@ import sys; assert sys.version_info[0] >= 3, "Python 3 required."
 
 import struct
 
-def write_compact_size(n):
+MAX_SIZE = 0x2000000
+
+def write_compact_size(n, allow_u64=False):
+    assert allow_u64 or n <= MAX_SIZE
     if n < 253:
         return struct.pack('B', n)
     elif n <= 0xFFFF:
@@ -13,7 +16,12 @@ def write_compact_size(n):
     else:
         return struct.pack('B', 255) + struct.pack('<Q', n)
 
-def parse_compact_size(rest):
+def parse_compact_size(rest, allow_u64=False):
+    (n, rest) = parse_compact_u64(rest)
+    assert allow_u64 or n <= MAX_SIZE
+    return (n, rest)
+
+def parse_compact_u64(rest):
     assert len(rest) >= 1
     b = rest[0]
     if b < 253:
@@ -35,36 +43,43 @@ def parse_compact_size(rest):
         return (n, rest[9:])
 
 
-def assert_parse_fails(encoding):
+def assert_parse_fails(encoding, allow_u64):
     try:
-        parse_compact_size(encoding)
+        parse_compact_size(encoding, allow_u64)
     except AssertionError:
         pass
     else:
         raise AssertionError("parse_compact_size(%r) failed to raise AssertionError" % (encoding,))
 
-def test_round_trip(n, encoding):
-    assert write_compact_size(n) == encoding
-    assert parse_compact_size(encoding) == (n, b'')
-    assert parse_compact_size(encoding + b'*') == (n, b'*')
-    assert_parse_fails(encoding[:-1])
+def test_round_trip(n, encoding, allow_u64):
+    assert write_compact_size(n, allow_u64) == encoding
+    assert parse_compact_size(encoding, allow_u64) == (n, b'')
+    assert parse_compact_size(encoding + b'*', allow_u64) == (n, b'*')
+    assert_parse_fails(encoding[:-1], allow_u64)
 
-test_round_trip(0, b'\x00')
-test_round_trip(1, b'\x01')
-test_round_trip(252, b'\xFC')
-test_round_trip(253, b'\xFD\xFD\x00')
-test_round_trip(254, b'\xFD\xFE\x00')
-test_round_trip(255, b'\xFD\xFF\x00')
-test_round_trip(256, b'\xFD\x00\x01')
-test_round_trip(0xFFFE, b'\xFD\xFE\xFF')
-test_round_trip(0xFFFF, b'\xFD\xFF\xFF')
-test_round_trip(0x010000, b'\xFE\x00\x00\x01\x00')
-test_round_trip(0x010001, b'\xFE\x01\x00\x01\x00')
-test_round_trip(0xFFFFFFFE, b'\xFE\xFE\xFF\xFF\xFF')
-test_round_trip(0xFFFFFFFF, b'\xFE\xFF\xFF\xFF\xFF')
-test_round_trip(0x0100000000, b'\xFF\x00\x00\x00\x00\x01\x00\x00\x00')
-test_round_trip(0xFFFFFFFFFFFFFFFF, b'\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF')
+for allow_u64 in (False, True):
+    test_round_trip(0, b'\x00', allow_u64)
+    test_round_trip(1, b'\x01', allow_u64)
+    test_round_trip(252, b'\xFC', allow_u64)
+    test_round_trip(253, b'\xFD\xFD\x00', allow_u64)
+    test_round_trip(254, b'\xFD\xFE\x00', allow_u64)
+    test_round_trip(255, b'\xFD\xFF\x00', allow_u64)
+    test_round_trip(256, b'\xFD\x00\x01', allow_u64)
+    test_round_trip(0xFFFE, b'\xFD\xFE\xFF', allow_u64)
+    test_round_trip(0xFFFF, b'\xFD\xFF\xFF', allow_u64)
+    test_round_trip(0x010000, b'\xFE\x00\x00\x01\x00', allow_u64)
+    test_round_trip(0x010001, b'\xFE\x01\x00\x01\x00', allow_u64)
+    test_round_trip(0x02000000, b'\xFE\x00\x00\x00\x02', allow_u64)
 
-assert_parse_fails(b'\xFD\xFC\x00')
-assert_parse_fails(b'\xFE\xFF\xFF\x00\x00')
-assert_parse_fails(b'\xFF\xFF\xFF\xFF\xFF\x00\x00\x00\x00')
+    assert_parse_fails(b'\xFD\xFC\x00', allow_u64)
+    assert_parse_fails(b'\xFE\xFF\xFF\x00\x00', allow_u64)
+    assert_parse_fails(b'\xFF\xFF\xFF\xFF\xFF\x00\x00\x00\x00', allow_u64)
+
+assert_parse_fails(b'\xFE\x01\x00\x00\x02', False)
+assert_parse_fails(b'\xFF\x00\x00\x00\x00\x01\x00\x00\x00', False)
+assert_parse_fails(b'\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF', False)
+
+test_round_trip(0xFFFFFFFE, b'\xFE\xFE\xFF\xFF\xFF', True)
+test_round_trip(0xFFFFFFFF, b'\xFE\xFF\xFF\xFF\xFF', True)
+test_round_trip(0x0100000000, b'\xFF\x00\x00\x00\x00\x01\x00\x00\x00', True)
+test_round_trip(0xFFFFFFFFFFFFFFFF, b'\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF', True)