diff --git a/src/highdicom/seg/content.py b/src/highdicom/seg/content.py
index 72208aa2..94b94435 100644
--- a/src/highdicom/seg/content.py
+++ b/src/highdicom/seg/content.py
@@ -481,8 +481,9 @@ def get_plane_positions_of_image(
             )
         elif self._coordinate_system == CoordinateSystemNames.SLIDE:
             if hasattr(image, 'PerFrameFunctionalGroupsSequence'):
-                plane_positions = [
+                plane_positions = [PlanePositionSequence.from_sequence(
                     item.PlanePositionSlideSequence
+                )
                     for item in image.PerFrameFunctionalGroupsSequence
                 ]
             else:
@@ -491,7 +492,7 @@ def get_plane_positions_of_image(
                 plane_positions = compute_plane_position_slide_per_frame(image)
         else:
             plane_positions = [
-                item.PlanePositionSequence
+                PlanePositionSequence.from_sequence(item.PlanePositionSequence)
                 for item in image.PerFrameFunctionalGroupsSequence
             ]
 
diff --git a/tests/test_seg.py b/tests/test_seg.py
index 85b363d2..0ae848c7 100644
--- a/tests/test_seg.py
+++ b/tests/test_seg.py
@@ -2512,6 +2512,30 @@ def test_spatial_positions_not_preserved(self):
         assert po_item.ImageOrientationSlide == list(image_orientation)
         self.check_dimension_index_vals(instance)
 
+    def test_get_plane_positions_of_image_patient(self):
+        seq = DimensionIndexSequence(
+            coordinate_system='PATIENT'
+        )
+        plane_positions = seq.get_plane_positions_of_image(self._ct_multiframe)
+        for position in plane_positions:
+            assert isinstance(position, PlanePositionSequence)
+
+    def test_get_plane_positions_of_image_slide(self):
+        seq = DimensionIndexSequence(
+            coordinate_system='SLIDE'
+        )
+        plane_positions = seq.get_plane_positions_of_image(self._sm_image)
+        for position in plane_positions:
+            assert isinstance(position, PlanePositionSequence)
+
+    def test_get_plane_positions_of_series(self):
+        seq = DimensionIndexSequence(
+            coordinate_system='PATIENT'
+        )
+        plane_positions = seq.get_plane_positions_of_series(self._ct_series)
+        for position in plane_positions:
+            assert isinstance(position, PlanePositionSequence)
+
 
 class TestSegmentationParsing():