diff --git a/heat/engine/parameters.py b/heat/engine/parameters.py
index 0447738157..4aced043bb 100644
--- a/heat/engine/parameters.py
+++ b/heat/engine/parameters.py
@@ -369,32 +369,32 @@ class StringParam(Parameter):
 class ParsedParameter(Parameter):
     """A template parameter with cached parsed value."""
 
-    __slots__ = ('parsed',)
+    __slots__ = ('_parsed',)
 
     def __init__(self, name, schema, value=None):
         super(ParsedParameter, self).__init__(name, schema, value)
-        self._update_parsed()
+        self._parsed = None
 
-    def set_default(self, value):
-        super(ParsedParameter, self).set_default(value)
-        self._update_parsed()
-
-    def _update_parsed(self):
-        if self.has_value():
-            if self.user_value is not None:
-                self.parsed = self.parse(self.user_value)
+    @property
+    def parsed(self):
+        if self._parsed is None:
+            if self.has_value():
+                if self.user_value is not None:
+                    self._parsed = self.parse(self.user_value)
+                else:
+                    self._parsed = self.parse(self.default())
             else:
-                self.parsed = self.parse(self.default())
+                self._parsed = self.default_parsed()
+        return self._parsed
 
 
 class CommaDelimitedListParam(ParsedParameter, collections.Sequence):
     """A template parameter of type "CommaDelimitedList"."""
 
-    __slots__ = ('parsed',)
+    __slots__ = tuple()
 
-    def __init__(self, name, schema, value=None):
-        self.parsed = []
-        super(CommaDelimitedListParam, self).__init__(name, schema, value)
+    def default_parsed(self):
+        return []
 
     def parse(self, value):
         # only parse when value is not already a list
@@ -436,11 +436,10 @@ class CommaDelimitedListParam(ParsedParameter, collections.Sequence):
 class JsonParam(ParsedParameter):
     """A template parameter who's value is map or list."""
 
-    __slots__ = ('parsed',)
+    __slots__ = tuple()
 
-    def __init__(self, name, schema, value=None):
-        self.parsed = {}
-        super(JsonParam, self).__init__(name, schema, value)
+    def default_parsed(self):
+        return {}
 
     def parse(self, value):
         try:
diff --git a/heat/tests/test_parameters.py b/heat/tests/test_parameters.py
index 66218cf6a7..4779ea4383 100644
--- a/heat/tests/test_parameters.py
+++ b/heat/tests/test_parameters.py
@@ -379,7 +379,7 @@ class ParameterTestSpecific(common.HeatTestCase):
         schema = {'Type': 'Json',
                   'ConstraintDescription': 'wibble'}
         val = {"foo": "bar", "not_json": len}
-        err = self.assertRaises(ValueError,
+        err = self.assertRaises(exception.StackValidationFailed,
                                 new_parameter, 'p', schema, val)
         self.assertIn('Value must be valid JSON', str(err))
 
@@ -397,7 +397,7 @@ class ParameterTestSpecific(common.HeatTestCase):
         schema = {'Type': 'Json',
                   'ConstraintDescription': 'wibble'}
         val = "I am not a map"
-        err = self.assertRaises(ValueError,
+        err = self.assertRaises(exception.StackValidationFailed,
                                 new_parameter, 'p', schema, val)
         self.assertIn('Value must be valid JSON', str(err))