diff --git a/src/geogenalg/application/generalize_water_areas.py b/src/geogenalg/application/generalize_water_areas.py
index 482590cf..cf5cddde 100644
--- a/src/geogenalg/application/generalize_water_areas.py
+++ b/src/geogenalg/application/generalize_water_areas.py
@@ -119,13 +119,15 @@ def _get_shoreline_gdf_segments_and_skip_coords(
         data: GeoDataFrame,
     ) -> tuple[GeoDataFrame, GeoDataFrame, MultiPoint]:
         shoreline_gdf = data.loc[data.geometry.geom_type == "LineString"].copy()
+        shoreline_gdf.geometry = shoreline_gdf.geometry.force_2d()
+
         if not shoreline_gdf.empty:
             # Extract out any points which are found in the input (Polygon)
             # data, but not the shoreline LineString data. This allows to
             # determine non-shoreline vertices (e.g. territorial water borders
             # in case of sea areas) and skip smoothing them later.
             segments = get_segments_in_polygon_boundary_but_not_in_lines(
-                data,
+                data.geometry.force_2d(),
                 shoreline_gdf,
             )
 
diff --git a/src/geogenalg/continuity.py b/src/geogenalg/continuity.py
index 8b8a16bb..207fd24f 100644
--- a/src/geogenalg/continuity.py
+++ b/src/geogenalg/continuity.py
@@ -9,7 +9,7 @@
 from warnings import warn
 
 import numpy as np
-from geopandas import GeoDataFrame
+from geopandas import GeoDataFrame, GeoSeries
 from networkx.classes.graph import Graph
 from scipy.sparse import csr_matrix
 from scipy.sparse.csgraph import connected_components
@@ -994,7 +994,7 @@ def add_contiguous_lines_information(  # noqa: PLR0913
 
 
 def get_segments_in_polygon_boundary_but_not_in_lines(
-    polygons: GeoDataFrame,
+    polygons: GeoDataFrame | GeoSeries,
     lines: GeoDataFrame,
 ) -> GeoDataFrame:
     """Find line segments present in polygons but not lines.
diff --git a/tests/testdata/algo/water_areas.gpkg b/tests/testdata/algo/water_areas.gpkg
index 1ba98650..540ea4a6 100644
Binary files a/tests/testdata/algo/water_areas.gpkg and b/tests/testdata/algo/water_areas.gpkg differ
diff --git a/tests/testdata/algo/watercourse_areas.gpkg b/tests/testdata/algo/watercourse_areas.gpkg
index 190e67c5..764f883f 100644
Binary files a/tests/testdata/algo/watercourse_areas.gpkg and b/tests/testdata/algo/watercourse_areas.gpkg differ