diff --git a/openmc/universe.py b/openmc/universe.py
index 4f36c3d85e..4054485fda 100644
--- a/openmc/universe.py
+++ b/openmc/universe.py
@@ -413,6 +413,7 @@ class Universe(UniverseBase):
                     colors="k",
                     linestyles="solid",
                     linewidths=1,
+                    levels=np.unique(image_value),
                     extent=(x_min, x_max, y_min, y_max),
                 )
 
diff --git a/tests/unit_tests/test_plots.py b/tests/unit_tests/test_plots.py
index ee3b2c1e27..0dddc827cc 100644
--- a/tests/unit_tests/test_plots.py
+++ b/tests/unit_tests/test_plots.py
@@ -12,7 +12,6 @@ def myplot():
     plot.filename = 'myplot'
     plot.type = 'slice'
     plot.basis = 'yz'
-    plot.outline = True
     plot.background = 'black'
     plot.background = (0, 0, 0)
 
diff --git a/tests/unit_tests/test_universe.py b/tests/unit_tests/test_universe.py
index a70bd46673..1bfbbf7ff8 100644
--- a/tests/unit_tests/test_universe.py
+++ b/tests/unit_tests/test_universe.py
@@ -59,6 +59,7 @@ def test_plot(run_in_tmpdir, sphere_model):
             pixels=(10, 10),
             color_by='material',
             colors=colors,
+            outline=True,
         )