diff --git a/pyFTS/common/transformations/roi.py b/pyFTS/common/transformations/roi.py
index 293fbf1..6e84497 100644
--- a/pyFTS/common/transformations/roi.py
+++ b/pyFTS/common/transformations/roi.py
@@ -1,4 +1,5 @@
 from pyFTS.common.transformations.transformation import Transformation 
+import numpy as np
 
 
 class ROI(Transformation):
@@ -20,4 +21,4 @@ class ROI(Transformation):
 
     def inverse(self, data, param=None, **kwargs):
         modified = [(param[i - 1] * data[i]) + param[i - 1] for i in np.arange(1, len(data))]
-        return modified
\ No newline at end of file
+        return modified