diff --git a/examples/plot_minimal_2d_example.py b/examples/plot_minimal_2d_example.py
index f290368..1af84b1 100644
--- a/examples/plot_minimal_2d_example.py
+++ b/examples/plot_minimal_2d_example.py
@@ -1,7 +1,7 @@
 import openmc
 from matplotlib.colors import LogNorm
-from openmc_regular_mesh_plotter import plot_mesh_tally
 
+from openmc_regular_mesh_plotter import plot_mesh_tally
 
 # MATERIALS
 mat_1 = openmc.Material()
diff --git a/examples/plot_minimal_example.py b/examples/plot_minimal_example.py
index 04342ee..f6bedaf 100644
--- a/examples/plot_minimal_example.py
+++ b/examples/plot_minimal_example.py
@@ -1,7 +1,7 @@
 import openmc
 from matplotlib.colors import LogNorm
-from openmc_regular_mesh_plotter import plot_mesh_tally
 
+from openmc_regular_mesh_plotter import plot_mesh_tally
 
 # MATERIALS
 mat_1 = openmc.Material()
diff --git a/examples/plot_sweep_through_slice_indexes.py b/examples/plot_sweep_through_slice_indexes.py
index 9c321f2..a3dd299 100644
--- a/examples/plot_sweep_through_slice_indexes.py
+++ b/examples/plot_sweep_through_slice_indexes.py
@@ -2,12 +2,13 @@
 # It is also set up to accept any geometry and the mesh tally will adapt to the geometry dimensions
 
 
-import openmc
+import matplotlib
 import numpy as np
+import openmc
+from matplotlib import cm
 from matplotlib.colors import LogNorm
+
 from openmc_regular_mesh_plotter import plot_mesh_tally
-from matplotlib import cm
-import matplotlib
 
 # sets the font for the axis
 matplotlib.rc("font", **{"family": "normal", "size": 22})
diff --git a/examples/plot_two_tallies_combined.py b/examples/plot_two_tallies_combined.py
index 10d6c87..fe6af27 100644
--- a/examples/plot_two_tallies_combined.py
+++ b/examples/plot_two_tallies_combined.py
@@ -1,7 +1,7 @@
 import openmc
 from matplotlib.colors import LogNorm
-from openmc_regular_mesh_plotter import plot_mesh_tally
 
+from openmc_regular_mesh_plotter import plot_mesh_tally
 
 # MATERIALS
 mat_1 = openmc.Material()
diff --git a/examples/plot_with_custom_color_map.py b/examples/plot_with_custom_color_map.py
index cdac48a..a5ae7af 100644
--- a/examples/plot_with_custom_color_map.py
+++ b/examples/plot_with_custom_color_map.py
@@ -2,12 +2,12 @@
 # It is also set up to accept any geometry and the mesh tally will adapt to the geometry dimensions
 
 
+import matplotlib.pyplot as plt
 import openmc
-from matplotlib.colors import LogNorm
-from openmc_regular_mesh_plotter import plot_mesh_tally
 from matplotlib import cm
-import matplotlib.pyplot as plt
+from matplotlib.colors import LogNorm
 
+from openmc_regular_mesh_plotter import plot_mesh_tally
 
 # materials
 mat_concrete = openmc.Material()
diff --git a/src/openmc_regular_mesh_plotter/__init__.py b/src/openmc_regular_mesh_plotter/__init__.py
index dd0f602..146eafb 100644
--- a/src/openmc_regular_mesh_plotter/__init__.py
+++ b/src/openmc_regular_mesh_plotter/__init__.py
@@ -1,7 +1,7 @@
 try:
-    from importlib.metadata import version, PackageNotFoundError
+    from importlib.metadata import PackageNotFoundError, version
 except (ModuleNotFoundError, ImportError):
-    from importlib_metadata import version, PackageNotFoundError
+    from importlib_metadata import PackageNotFoundError, version
 try:
     __version__ = version("openmc_regular_mesh_plotter")
 except PackageNotFoundError:
diff --git a/src/openmc_regular_mesh_plotter/core.py b/src/openmc_regular_mesh_plotter/core.py
index b42b905..0d3264b 100644
--- a/src/openmc_regular_mesh_plotter/core.py
+++ b/src/openmc_regular_mesh_plotter/core.py
@@ -1,13 +1,12 @@
 import math
+import typing
 from pathlib import Path
 from tempfile import TemporaryDirectory
-import typing
-import openmc
+
+import matplotlib.pyplot as plt
 import numpy as np
 import openmc
 import openmc.checkvalue as cv
-import matplotlib.pyplot as plt
-
 from packaging import version
 
 if version.parse(openmc.__version__) < version.parse("0.13.3"):
@@ -182,6 +181,13 @@ def plot_mesh_tally(
     if colorbar:
         fig.colorbar(im, **colorbar_kwargs)
 
+    if outline and geometry is None:
+        msg = (
+            "When calling plot_mesh_tally with outline=True the geometry "
+            "should also be provided. Either set outline to False or set "
+            "the geometry to and openmc.Geometry object"
+        )
+        raise ValueError(msg)
     if outline and geometry is not None:
         import matplotlib.image as mpimg
 
diff --git a/tests/test_units.py b/tests/test_units.py
index df33329..e6b9826 100644
--- a/tests/test_units.py
+++ b/tests/test_units.py
@@ -1,7 +1,8 @@
 import openmc
+import pytest
 from matplotlib.colors import LogNorm
+
 from openmc_regular_mesh_plotter import plot_mesh_tally
-import pytest
 
 
 @pytest.fixture()