diff --git a/design/mvp/CanonicalABI.md b/design/mvp/CanonicalABI.md
index 16f3164d..0f2648ce 100644
--- a/design/mvp/CanonicalABI.md
+++ b/design/mvp/CanonicalABI.md
@@ -180,6 +180,7 @@ def size_variant(cases):
 
 def size_flags(labels):
   n = len(labels)
+  if n == 0: return 0
   if n <= 8: return 1
   if n <= 16: return 2
   return 4 * num_i32_flags(labels)
diff --git a/design/mvp/canonical-abi/definitions.py b/design/mvp/canonical-abi/definitions.py
index 3a90ec58..3d09ef21 100644
--- a/design/mvp/canonical-abi/definitions.py
+++ b/design/mvp/canonical-abi/definitions.py
@@ -183,6 +183,7 @@ def size_variant(cases):
 
 def size_flags(labels):
   n = len(labels)
+  if n == 0: return 0
   if n <= 8: return 1
   if n <= 16: return 2
   return 4 * num_i32_flags(labels)
diff --git a/design/mvp/canonical-abi/run_tests.py b/design/mvp/canonical-abi/run_tests.py
index e6586375..965c0246 100644
--- a/design/mvp/canonical-abi/run_tests.py
+++ b/design/mvp/canonical-abi/run_tests.py
@@ -95,6 +95,8 @@ def test_name():
 test(Unit(), [], {})
 test(Record([Field('x',U8()), Field('y',U16()), Field('z',U32())]), [1,2,3], {'x':1,'y':2,'z':3})
 test(Tuple([Tuple([U8(),U8()]),U8()]), [1,2,3], {'0':{'0':1,'1':2},'1':3})
+t = Flags([])
+test(t, [], {})
 t = Flags(['a','b'])
 test(t, [0], {'a':False,'b':False})
 test(t, [2], {'a':False,'b':True})
@@ -283,6 +285,12 @@ def test_heap(t, expect, args, byte_array):
           [1,0xff,5,0,6,0xff,7,0xff,  0,0xff,8,0xff,0xff,0xff,9,0xff])
 test_heap(List(Union([U8()])), [{'0':6},{'0':7},{'0':8}], [0,3],
           [0,6, 0,7, 0,8])
+t = List(Flags([]))
+test_heap(t, [{},{},{}], [0,3],
+          [])
+t = List(Tuple([Flags([]), U8()]))
+test_heap(t, [mk_tup({}, 42), mk_tup({}, 43), mk_tup({}, 44)], [0,3],
+          [42,43,44])
 t = List(Flags(['a','b']))
 test_heap(t, [{'a':False,'b':False},{'a':False,'b':True},{'a':True,'b':True}], [0,3],
           [0,2,3])