diff --git a/src/h3/_h3shape.py b/src/h3/_h3shape.py
index 7aa7f628..a8146df0 100644
--- a/src/h3/_h3shape.py
+++ b/src/h3/_h3shape.py
@@ -301,7 +301,11 @@ def geo_to_h3shape(geo):
         # get dict
         geo = geo.__geo_interface__
 
-    assert isinstance(geo, dict)  # todo: remove
+    if not isinstance(geo, dict):
+        raise ValueError(
+            'geo must be a dict or implement __geo_interface__, got: '
+            + str(type(geo))
+        )
 
     t = geo['type']
     coord = geo['coordinates']
diff --git a/tests/test_lib/polyfill/test_h3.py b/tests/test_lib/polyfill/test_h3.py
index 72ddd586..fed297f8 100644
--- a/tests/test_lib/polyfill/test_h3.py
+++ b/tests/test_lib/polyfill/test_h3.py
@@ -302,6 +302,11 @@ def test_geo_to_h3shape_passthrough():
         assert h3.geo_to_h3shape(shape) is shape
 
 
+def test_geo_to_h3shape_rejects_non_dict():
+    with pytest.raises(ValueError, match='dict or implement __geo_interface__'):
+        h3.geo_to_h3shape('not a geo object')
+
+
 def test_polyfill_down_under():
     sydney = [
         (-33.8556, 151.1979),