diff --git a/src/client/Ruby/ms-rest/lib/ms_rest/serialization.rb b/src/client/Ruby/ms-rest/lib/ms_rest/serialization.rb
index c76eae2797485..2998c0da6f3d7 100644
--- a/src/client/Ruby/ms-rest/lib/ms_rest/serialization.rb
+++ b/src/client/Ruby/ms-rest/lib/ms_rest/serialization.rb
@@ -315,7 +315,12 @@ def serialize_composite_type(mapper, object, object_name)
 
         unless model_props.nil?
           model_props.each do |key, value|
-            instance_variable = object.instance_variable_get("@#{key}")
+            begin
+              instance_variable = object.instance_variable_get("@#{key}")
+            rescue NameError
+              fail ValidationError, "instance variable '#{key}' is expected on '#{object.class}'."
+            end
+
             if !instance_variable.nil? && instance_variable.respond_to?(:validate)
               instance_variable.validate
             end