diff --git a/test/test_hexahedron.py b/test/test_hexahedron.py
index <HASH>..<HASH> 100644
--- a/test/test_hexahedron.py
+++ b/test/test_hexahedron.py
@@ -56,7 +56,7 @@ def test_normality(n=4):
 
 def test_write():
     orthopy.hexahedron.write(
-        "hexa.vtu", lambda X: orthopy.hexahedron.tree(X.T, 5)[5][5]
+        "hexa.vtk", lambda X: orthopy.hexahedron.tree(X.T, 5)[5][5]
     )
     return
 
diff --git a/test/test_sphere_sph.py b/test/test_sphere_sph.py
index <HASH>..<HASH> 100644
--- a/test/test_sphere_sph.py
+++ b/test/test_sphere_sph.py
@@ -163,7 +163,7 @@ def test_write():
         out = abs(out)
         return out
 
-    orthopy.sphere.write("sph.vtu", sph22)
+    orthopy.sphere.write("sph.vtk", sph22)
     return
test: vtu -> vtk