diff --git a/pylot/core/active/fmtomo2vtk.py b/pylot/core/active/fmtomo2vtk.py
index 2c35958a..8de95354 100644
--- a/pylot/core/active/fmtomo2vtk.py
+++ b/pylot/core/active/fmtomo2vtk.py
@@ -1,3 +1,5 @@
+import numpy as np
+
 def vgrids2VTK(inputfile = 'vgrids.in', outputfile = 'vgrids.vtk'):
     '''
     Generate a vtk-file readable by e.g. paraview from FMTOMO output vgrids.in