diff --git a/kwant/plotter.py b/kwant/plotter.py
index a3c4320fc0512b8eba914c275f0f1652a148860e..4fcdbacd69c9beee0b0aacb4dcc61ff7377add5c 100644
--- a/kwant/plotter.py
+++ b/kwant/plotter.py
@@ -1943,7 +1943,7 @@ def interpolate_current(syst, current, relwidth=None, abswidth=None, n=9):
             # Zero volume: nothing to do.
             continue
 
-        field_slice = [slice(*slices[i, d]) for d in range(dim)]
+        field_slice = tuple([slice(*slices[i, d]) for d in range(dim)])
 
         # Coordinates of the grid points that are within range of the current
         # hopping.