diff --git a/contrib/pyln-proto/pyln/proto/onion.py b/contrib/pyln-proto/pyln/proto/onion.py
index 9c9e9945d..eb35c9aa8 100644
--- a/contrib/pyln-proto/pyln/proto/onion.py
+++ b/contrib/pyln-proto/pyln/proto/onion.py
@@ -184,11 +184,27 @@ class TlvField(object):
 
 
 class Tu32Field(TlvField):
-    pass
+    def to_bytes(self):
+        raw = struct.pack("!I", self.value)
+        while len(raw) > 1 and raw[0] == 0:
+            raw = raw[1:]
+        b = BytesIO()
+        varint_encode(self.typenum, b)
+        varint_encode(len(raw), b)
+        b.write(raw)
+        return b.getvalue()
 
 
 class Tu64Field(TlvField):
-    pass
+    def to_bytes(self):
+        raw = struct.pack("!Q", self.value)
+        while len(raw) > 1 and raw[0] == 0:
+            raw = raw[1:]
+        b = BytesIO()
+        varint_encode(self.typenum, b)
+        varint_encode(len(raw), b)
+        b.write(raw)
+        return b.getvalue()
 
 
 class ShortChannelIdField(TlvField):
diff --git a/contrib/pyln-proto/tests/test_onion.py b/contrib/pyln-proto/tests/test_onion.py
index b45c41fe5..62d9da907 100644
--- a/contrib/pyln-proto/tests/test_onion.py
+++ b/contrib/pyln-proto/tests/test_onion.py
@@ -30,3 +30,31 @@ def test_tlv_payload():
     ))
 
     assert(payload.to_bytes() == tlv)
+
+
+def test_tu_fields():
+    pairs = [
+        (0, b'\x01\x01\x00'),
+        (1 << 8, b'\x01\x02\x01\x00'),
+        (1 << 16, b'\x01\x03\x01\x00\x00'),
+        (1 << 24, b'\x01\x04\x01\x00\x00\x00'),
+        ((1 << 32) - 1, b'\x01\x04\xFF\xFF\xFF\xFF'),
+    ]
+
+    # These should work for Tu32
+    for i, o in pairs:
+        f = onion.Tu32Field(1, i)
+        assert(f.to_bytes() == o)
+
+    # And these should work for Tu64
+    pairs += [
+        (1 << 32, b'\x01\x05\x01\x00\x00\x00\x00'),
+        (1 << 40, b'\x01\x06\x01\x00\x00\x00\x00\x00'),
+        (1 << 48, b'\x01\x07\x01\x00\x00\x00\x00\x00\x00'),
+        (1 << 56, b'\x01\x08\x01\x00\x00\x00\x00\x00\x00\x00'),
+        ((1 << 64) - 1, b'\x01\x08\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF'),
+    ]
+
+    for i, o in pairs:
+        f = onion.Tu64Field(1, i)
+        assert(f.to_bytes() == o)