diff --git a/lib/errors/var_not_defined.rb b/lib/errors/var_not_defined.rb
new file mode 100644
index 0000000..11c0ed5
--- /dev/null
+++ b/lib/errors/var_not_defined.rb
@@ -0,0 +1,7 @@
+module Mm2ep
+  module Depend
+    class VarNotDefined < Depend::EvalError
+    
+    end
+  end
+end
diff --git a/lib/mm2ep_depend.rb b/lib/mm2ep_depend.rb
index 77d76bc..ef6197f 100644
--- a/lib/mm2ep_depend.rb
+++ b/lib/mm2ep_depend.rb
@@ -8,3 +8,5 @@ require 'rly'
 
 require 'mm2ep_depend/lexer'
 require 'mm2ep_depend/parser'
+require 'mm2ep_depend/eval_error'
+require 'errors/var_not_defined'
diff --git a/lib/mm2ep_depend/eval_error.rb b/lib/mm2ep_depend/eval_error.rb
new file mode 100644
index 0000000..df75bcb
--- /dev/null
+++ b/lib/mm2ep_depend/eval_error.rb
@@ -0,0 +1,18 @@
+module Mm2ep
+  module Depend
+    class EvalError
+      attr_accessor :var
+      attr_accessor :message
+
+      def initialize(hash)
+        # validate input
+        raise ArgumentError unless hash[:message]
+
+        # load input
+        @message = hash[:message]
+        @var = hash[:var]
+      end
+
+    end
+  end
+end