diff --git a/capnp/lib/capnp.pyx b/capnp/lib/capnp.pyx
index 4c0183d..42f96c0 100644
--- a/capnp/lib/capnp.pyx
+++ b/capnp/lib/capnp.pyx
@@ -1393,6 +1393,8 @@ cdef class _DynamicStructBuilder:
 
         :Raises: :exc:`KjException` if the field isn't in this struct
         """
+        if isinstance(field, _StructModuleWhich):
+            field = field.name[0].lower() + field.name[1:]
         if size is None:
             return to_python_builder(self.thisptr.init(field), self._parent)
         else:
diff --git a/test/test_struct.py b/test/test_struct.py
index b1bff8b..8dca0fd 100644
--- a/test/test_struct.py
+++ b/test/test_struct.py
@@ -217,6 +217,9 @@ def test_union_enum(all_types):
     msg = all_types.GroupedUnionAllTypes.new_message(**{'g2': {'unionStructField2': {'textField': "foo"}}})
     assert msg.which == all_types.GroupedUnionAllTypes.Union.G2
 
+    msg = all_types.UnionAllTypes.new_message()
+    msg.unionStructField2 = msg.init(all_types.UnionAllTypes.Union.UnionStructField2)
+
 
 def isstr(s):
     return isinstance(s, str)