diff --git a/python/mocha/oms/Oms.py b/python/mocha/oms/Oms.py
index dbddddf..204dab8 100644
--- a/python/mocha/oms/Oms.py
+++ b/python/mocha/oms/Oms.py
@@ -56,6 +56,9 @@ class Oms:
 
 	def get_attribute_value(self, inst : InstanceReference, att : InstanceReference, default_value = None):
 		value = self.get_attribute_value_internal(inst, att, default_value)
+		if value is None:
+			return default_value
+
 		pclass = self.get_parent_class(att)
 		if pclass is not None:
 			if pclass.get_global_identifier() == KnownClassGuids.NumericAttribute:
@@ -79,7 +82,7 @@ class Oms:
 					max_val = self.get_attribute_value(att, self.get_instance_by_global_identifier(KnownAttributeGuids.MaximumValue))
 					min_val = self.get_attribute_value(att, self.get_instance_by_global_identifier(KnownAttributeGuids.MinimumValue))
 					
-					if val < min_val or val > max_val:
+					if (min_val is not None and val < min_val) or (max_val is not None and val > max_val):
 						return OmsResult(False, "parameter out of range: value")
 
 				except ValueError: