diff --git a/xrspatial/aspect.py b/xrspatial/aspect.py
index a0fd626c5..8155d029e 100644
--- a/xrspatial/aspect.py
+++ b/xrspatial/aspect.py
@@ -445,7 +445,7 @@ def aspect(agg: xr.DataArray,
     else:  # geodesic
         if z_unit not in Z_UNITS:
             raise ValueError(
-                f"z_unit must be one of {sorted(set(Z_UNITS.values()), key=str)}, "
+                f"z_unit must be one of {sorted(Z_UNITS)}, "
                 f"got {z_unit!r}"
             )
         z_factor = Z_UNITS[z_unit]
diff --git a/xrspatial/slope.py b/xrspatial/slope.py
index 74bc06b8c..85a857f64 100644
--- a/xrspatial/slope.py
+++ b/xrspatial/slope.py
@@ -412,7 +412,7 @@ def slope(agg: xr.DataArray,
     else:  # geodesic
         if z_unit not in Z_UNITS:
             raise ValueError(
-                f"z_unit must be one of {sorted(set(Z_UNITS.values()), key=str)}, "
+                f"z_unit must be one of {sorted(Z_UNITS)}, "
                 f"got {z_unit!r}"
             )
         z_factor = Z_UNITS[z_unit]
diff --git a/xrspatial/tests/test_geodesic_aspect.py b/xrspatial/tests/test_geodesic_aspect.py
index fcf96163c..b661bc292 100644
--- a/xrspatial/tests/test_geodesic_aspect.py
+++ b/xrspatial/tests/test_geodesic_aspect.py
@@ -1,4 +1,6 @@
 """Tests for geodesic aspect computation."""
+import re
+
 import numpy as np
 import pytest
 import xarray as xr
@@ -213,9 +215,17 @@ def test_invalid_method_raises(self):
     def test_invalid_z_unit_raises(self):
         elev = _flat_surface()
         raster = _make_geo_raster(elev, 40.0, 41.0, 10.0, 11.0)
-        with pytest.raises(ValueError, match="z_unit"):
+        with pytest.raises(ValueError, match="z_unit") as excinfo:
             aspect(raster, method='geodesic', z_unit='cubit')
 
+        # The message must list the accepted unit-name strings (the keys a
+        # user is allowed to pass), not the numeric conversion factors.
+        msg = str(excinfo.value)
+        assert "'meter'" in msg
+        assert "'foot'" in msg
+        # No bare numeric conversion factor should leak into the message.
+        assert not re.search(r"\d+\.\d+", msg)
+
     def test_missing_coords_raises(self):
         data = np.ones((5, 5))
         raster = xr.DataArray(data, dims=['dim_0', 'dim_1'])
diff --git a/xrspatial/tests/test_geodesic_slope.py b/xrspatial/tests/test_geodesic_slope.py
index 2747f95f5..64bb4240e 100644
--- a/xrspatial/tests/test_geodesic_slope.py
+++ b/xrspatial/tests/test_geodesic_slope.py
@@ -1,4 +1,6 @@
 """Tests for geodesic slope computation."""
+import re
+
 import numpy as np
 import pytest
 import xarray as xr
@@ -209,9 +211,17 @@ def test_invalid_method_raises(self):
     def test_invalid_z_unit_raises(self):
         elev = _flat_surface()
         raster = _make_geo_raster(elev, 40.0, 41.0, 10.0, 11.0)
-        with pytest.raises(ValueError, match="z_unit"):
+        with pytest.raises(ValueError, match="z_unit") as excinfo:
             slope(raster, method='geodesic', z_unit='cubit')
 
+        # The message must list the accepted unit-name strings (the keys a
+        # user is allowed to pass), not the numeric conversion factors.
+        msg = str(excinfo.value)
+        assert "'meter'" in msg
+        assert "'foot'" in msg
+        # No bare numeric conversion factor should leak into the message.
+        assert not re.search(r"\d+\.\d+", msg)
+
     def test_missing_coords_raises(self):
         data = np.ones((5, 5))
         raster = xr.DataArray(data, dims=['dim_0', 'dim_1'])