diff --git a/core/utils/descriptors/evaluated.py b/core/utils/descriptors/evaluated.py
index 0e1b687..4f57682 100644
--- a/core/utils/descriptors/evaluated.py
+++ b/core/utils/descriptors/evaluated.py
@@ -66,8 +66,9 @@ class Evaluated(object):
     def __set__(self, instance, value):
         attribs = instance.__dict__
         value = value or self.default
-        if isinstance(value, six.text_type) and value.startswith('${') and value.endswith('}'):
+        if isinstance(value, six.string_types) and value.startswith('${') and value.endswith('}'):
             attribs[self.name_raw] = value[2:-1].strip()
+            attribs.pop(self.name, None)  # reset previous eval result
         else:
             attribs[self.name] = type(self.default)(value)
 
diff --git a/tests/test_evaled_property.py b/tests/test_evaled_property.py
index 27957cd..468f16b 100644
--- a/tests/test_evaled_property.py
+++ b/tests/test_evaled_property.py
@@ -56,6 +56,8 @@ def test_evaled():
     assert a.foo == 11 and a.foo == 11
     assert a.called['foo'] == 2
     assert not a.errors
+    a.foo = u'${ 10 + 2 }'
+    assert a.foo == 12
 
 
 def test_evaled_with_default():