diff --git a/contrib/pyln-client/tests/test_units.py b/contrib/pyln-client/tests/test_units.py
index 049133803..582f918d9 100644
--- a/contrib/pyln-client/tests/test_units.py
+++ b/contrib/pyln-client/tests/test_units.py
@@ -53,11 +53,221 @@ def test_floats():
     # test floating point arithmetic
     amount = Millisatoshi("1000msat") * 0.1
     assert int(amount) == 100
+    amount = Millisatoshi("100msat") * 0.1
+    assert int(amount) == 10
+    amount = Millisatoshi("10msat") * 0.1
+    assert int(amount) == 1
 
+
+def test_zero():
+    # zero amounts are of course valid
+    amount = Millisatoshi("0btc")
+    assert int(amount) == 0
+    amount = Millisatoshi("0sat")
+    assert int(amount) == 0
+    amount = Millisatoshi("0msat")
+    assert int(amount) == 0
+
+    # zero floating amount as well
+    amount = Millisatoshi("0.0btc")
+    assert int(amount) == 0
+    amount = Millisatoshi("0.0sat")
+    assert int(amount) == 0
+    amount = Millisatoshi("0.0msat")
+    assert int(amount) == 0
+
+    # also anything multiplied by zero
+    amount = Millisatoshi("1btc") * 0
+    assert int(amount) == 0
+    amount = Millisatoshi("1sat") * 0
+    assert int(amount) == 0
+    amount = Millisatoshi("1msat") * 0
+    assert int(amount) == 0
+
+    # and multiplied by a floating zero
+    amount = Millisatoshi("1btc") * 0.0
+    assert int(amount) == 0
+    amount = Millisatoshi("1sat") * 0.0
+    assert int(amount) == 0
+    amount = Millisatoshi("1msat") * 0.0
+    assert int(amount) == 0
+
+
+@pytest.mark.xfail
+def test_round_zero():
+    # everything below 1msat should round down to zero
+    amount = Millisatoshi("1msat") * 0.9
+    assert int(amount) == 0
+    amount = Millisatoshi("10msat") * 0.09
+    assert int(amount) == 0
+    amount = Millisatoshi("100msat") * 0.009
+    assert int(amount) == 0
+    amount = Millisatoshi("1000msat") * 0.0009
+    assert int(amount) == 0
+
+    amount = Millisatoshi("1sat") * 0.0009
+    assert int(amount) == 0
+    amount = Millisatoshi("0.1sat") * 0.009
+    assert int(amount) == 0
+    amount = Millisatoshi("0.01sat") * 0.09
+    assert int(amount) == 0
+    amount = Millisatoshi("0.001sat") * 0.9
+    assert int(amount) == 0
+
+    amount = Millisatoshi("10sat") * 0.00009
+    assert int(amount) == 0
+    amount = Millisatoshi("100sat") * 0.000009
+    assert int(amount) == 0
+    amount = Millisatoshi("1000sat") * 0.0000009
+    assert int(amount) == 0
+    amount = Millisatoshi("10000sat") * 0.00000009
+    assert int(amount) == 0
+    amount = Millisatoshi("10000sat") * 0.00000009
+    assert int(amount) == 0
+
+    amount = Millisatoshi("1btc") * 0.000000000009
+    assert int(amount) == 0
+    amount = Millisatoshi("0.1btc") * 0.00000000009
+    assert int(amount) == 0
+    amount = Millisatoshi("0.01btc") * 0.0000000009
+    assert int(amount) == 0
+    amount = Millisatoshi("0.001btc") * 0.000000009
+    assert int(amount) == 0
+    amount = Millisatoshi("0.0001btc") * 0.00000009
+    assert int(amount) == 0
+    amount = Millisatoshi("0.00001btc") * 0.0000009
+    assert int(amount) == 0
+    amount = Millisatoshi("0.000001btc") * 0.000009
+    assert int(amount) == 0
+    amount = Millisatoshi("0.0000001btc") * 0.00009
+    assert int(amount) == 0
+    amount = Millisatoshi("0.00000001btc") * 0.0009
+    assert int(amount) == 0
+    amount = Millisatoshi("0.000000001btc") * 0.009
+    assert int(amount) == 0
+    amount = Millisatoshi("0.0000000001btc") * 0.09
+    assert int(amount) == 0
+    amount = Millisatoshi("0.00000000001btc") * 0.9
+    assert int(amount) == 0
+
+
+@pytest.mark.xfail
+def test_round_down():
+    # sub msat significatns should be floored
+    amount = Millisatoshi("2msat") * 0.9
+    assert int(amount) == 1
+    amount = Millisatoshi("20msat") * 0.09
+    assert int(amount) == 1
+    amount = Millisatoshi("200msat") * 0.009
+    assert int(amount) == 1
+    amount = Millisatoshi("2000msat") * 0.0009
+    assert int(amount) == 1
+
+    amount = Millisatoshi("2sat") * 0.0009
+    assert int(amount) == 1
+    amount = Millisatoshi("0.2sat") * 0.009
+    assert int(amount) == 1
+    amount = Millisatoshi("0.02sat") * 0.09
+    assert int(amount) == 1
+    amount = Millisatoshi("0.002sat") * 0.9
+    assert int(amount) == 1
+
+    amount = Millisatoshi("20sat") * 0.00009
+    assert int(amount) == 1
+    amount = Millisatoshi("200sat") * 0.000009
+    assert int(amount) == 1
+    amount = Millisatoshi("2000sat") * 0.0000009
+    assert int(amount) == 1
+    amount = Millisatoshi("20000sat") * 0.00000009
+    assert int(amount) == 1
+    amount = Millisatoshi("20000sat") * 0.00000009
+    assert int(amount) == 1
+
+    amount = Millisatoshi("2btc") * 0.000000000009
+    assert int(amount) == 1
+    amount = Millisatoshi("0.2btc") * 0.00000000009
+    assert int(amount) == 1
+    amount = Millisatoshi("0.02btc") * 0.0000000009
+    assert int(amount) == 1
+    amount = Millisatoshi("0.002btc") * 0.000000009
+    assert int(amount) == 1
+    amount = Millisatoshi("0.0002btc") * 0.00000009
+    assert int(amount) == 1
+    amount = Millisatoshi("0.00002btc") * 0.0000009
+    assert int(amount) == 1
+    amount = Millisatoshi("0.000002btc") * 0.000009
+    assert int(amount) == 1
+    amount = Millisatoshi("0.0000002btc") * 0.00009
+    assert int(amount) == 1
+    amount = Millisatoshi("0.00000002btc") * 0.0009
+    assert int(amount) == 1
+    amount = Millisatoshi("0.000000002btc") * 0.009
+    assert int(amount) == 1
+    amount = Millisatoshi("0.0000000002btc") * 0.09
+    assert int(amount) == 1
+    amount = Millisatoshi("0.00000000002btc") * 0.9
+    assert int(amount) == 1
+
+
+def test_nosubmsat():
     # sub millisatoshi are not a concept yet
     with pytest.raises(ValueError, match='Millisatoshi must be a whole number'):
-        amount = Millisatoshi("0.000000000001btc")
+        Millisatoshi("0.1msat")
     with pytest.raises(ValueError, match='Millisatoshi must be a whole number'):
-        amount = Millisatoshi("0.0001sat")
+        Millisatoshi(".1msat")
     with pytest.raises(ValueError, match='Millisatoshi must be a whole number'):
-        amount = Millisatoshi("0.1msat")
+        Millisatoshi("0.0001sat")
+    with pytest.raises(ValueError, match='Millisatoshi must be a whole number'):
+        Millisatoshi(".0001sat")
+    with pytest.raises(ValueError, match='Millisatoshi must be a whole number'):
+        Millisatoshi("0.000000000001btc")
+    with pytest.raises(ValueError, match='Millisatoshi must be a whole number'):
+        Millisatoshi(".000000000001btc")
+
+
+def test_nonegative():
+    with pytest.raises(ValueError, match='Millisatoshi must be >= 0'):
+        Millisatoshi("-1btc")
+    with pytest.raises(ValueError, match='Millisatoshi must be >= 0'):
+        Millisatoshi("-1.0btc")
+    with pytest.raises(ValueError, match='Millisatoshi must be >= 0'):
+        Millisatoshi("-0.1btc")
+    with pytest.raises(ValueError, match='Millisatoshi must be >= 0'):
+        Millisatoshi("-.1btc")
+    with pytest.raises(ValueError, match='Millisatoshi must be >= 0'):
+        Millisatoshi("-1sat")
+    with pytest.raises(ValueError, match='Millisatoshi must be >= 0'):
+        Millisatoshi("-1.0sat")
+    with pytest.raises(ValueError, match='Millisatoshi must be >= 0'):
+        Millisatoshi("-0.1sat")
+    with pytest.raises(ValueError, match='Millisatoshi must be >= 0'):
+        Millisatoshi("-.1sat")
+    with pytest.raises(ValueError, match='Millisatoshi must be >= 0'):
+        Millisatoshi("-1msat")
+    with pytest.raises(ValueError, match='Millisatoshi must be >= 0'):
+        Millisatoshi("-1.0msat")
+
+    with pytest.raises(ValueError, match='Millisatoshi must be >= 0'):
+        Millisatoshi("1msat") * -1
+    with pytest.raises(ValueError, match='Millisatoshi must be >= 0'):
+        Millisatoshi("1msat") * -42
+    with pytest.raises(ValueError, match='Millisatoshi must be >= 0'):
+        Millisatoshi("1sat") * -1
+    with pytest.raises(ValueError, match='Millisatoshi must be >= 0'):
+        Millisatoshi("1btc") * -1
+
+    with pytest.raises(ValueError, match='Millisatoshi must be >= 0'):
+        Millisatoshi("1msat") / -1
+    with pytest.raises(ValueError, match='Millisatoshi must be >= 0'):
+        Millisatoshi("1msat") / -0.5
+    with pytest.raises(ValueError, match='Millisatoshi must be >= 0'):
+        Millisatoshi("1sat") / -1
+    with pytest.raises(ValueError, match='Millisatoshi must be >= 0'):
+        Millisatoshi("1btc") / -1
+
+    with pytest.raises(ValueError, match='Millisatoshi must be >= 0'):
+        Millisatoshi("1msat") // -1
+    with pytest.raises(ValueError, match='Millisatoshi must be >= 0'):
+        Millisatoshi("1sat") // -1
+    with pytest.raises(ValueError, match='Millisatoshi must be >= 0'):
+        Millisatoshi("1btc") // -1