diff --git a/pyFTS/common/transformations/differential.py b/pyFTS/common/transformations/differential.py
index 9952965..6a135ac 100644
--- a/pyFTS/common/transformations/differential.py
+++ b/pyFTS/common/transformations/differential.py
@@ -1,3 +1,4 @@
+import numpy as np
 from pyFTS.common.transformations.transformation import Transformation 
 
 
@@ -77,4 +78,4 @@ class Differential(Transformation):
         if n == 1:
             return inc[0]
         else:
-            return inc
\ No newline at end of file
+            return inc