diff --git a/composer/schema/field.py b/composer/schema/field.py
index 6bdc9ee..379e51c 100644
--- a/composer/schema/field.py
+++ b/composer/schema/field.py
@@ -144,6 +144,9 @@ class FieldInstance(object):
                 return method()
         return self.context.defaultValue
 
+    def getRawValue(self, data, key, default=None):
+        return data.get(key, default)
+
     def marshall(self, value):
         return value or u''
         #return toStr(value)
diff --git a/composer/schema/instance.py b/composer/schema/instance.py
index 1aec4b7..8caf2cd 100644
--- a/composer/schema/instance.py
+++ b/composer/schema/instance.py
@@ -85,7 +85,9 @@ class Editor(BaseInstance):
             name = f.name
             ftype = f.fieldType
             fi = formState.fieldInstances[name]
-            value = fi.unmarshall(data.get(name, u''))
+            #rawValue = data.get(name, u'')
+            rawValue = fi.getRawValue(data, name, u'')
+            value = fi.unmarshall(rawValue)
             if ftype in fieldHandlers:  # caller wants special treatment of field
                 fieldHandlers[ftype](context, value, fi, formState)
             else:
@@ -104,7 +106,8 @@ class Editor(BaseInstance):
             if f.readonly:
                 continue
             fi = f.getFieldInstance(self)
-            value = data.get(f.name)
+            #value = data.get(f.name)
+            value = fi.getRawValue(data, f.name)
             fi.validate(value, data)
             formState.fieldInstances.append(fi)
             formState.severity = max(formState.severity, fi.severity)
diff --git a/composer/schema/interfaces.py b/composer/schema/interfaces.py
index 9d7106e..9fd64c5 100644
--- a/composer/schema/interfaces.py
+++ b/composer/schema/interfaces.py
@@ -204,19 +204,25 @@ class IFieldInstance(Interface):
     severity = Attribute("An integer giving a state or error "
                     "code, 0 meaning 'OK'.")
 
+    def getRawValue(data, key, default=None):
+        """ Extract a raw value for the field from the data given
+            using the key; if no corresponding value is found return
+            the default.
+        """
+
     def marshall(value):
         """ Return a string (possibly unicode) representation of the
             value given that may be used for editing.
         """
 
     def display(value):
-        """ Return a string (possibly unicode) representation of the
+        """ Return a string- (possibly unicode-) based representation of the
             value given that may be used for presentation.
         """
 
-    def unmarshall(strValue):
-        """ Return the internal (real) value corresponding to the string
-            value given.
+    def unmarshall(rawValue):
+        """ Return the internal (real) value corresponding to the
+            raw value given.
         """
 
     def validate(value, data=None):