diff --git a/src/openflexure_microscope_server/things/stage_measure.py b/src/openflexure_microscope_server/things/stage_measure.py
index f9e2c14c..9dcce848 100644
--- a/src/openflexure_microscope_server/things/stage_measure.py
+++ b/src/openflexure_microscope_server/things/stage_measure.py
@@ -61,7 +61,7 @@ class RomDataTracker:
         self.offsets: list[dict[str, float]] = []
 
     def record_movement(
-        self, current_pos: dict[str, int], offset: dict[str, int]
+        self, current_pos: dict[str, int], offset: dict[str, float]
     ) -> None:
         """Record the current position and the measured offset of the last move."""
         self.stage_coords.append(current_pos)
@@ -76,7 +76,7 @@ class RomDataTracker:
         self,
         movement: dict[str, int],
         stage_position: dict[str, int],
-        csm_matrix: np.ndarray,
+        csm_matrix: list[list[int]],
     ) -> int:
         """Predict the z-displacement needed for a given movement.
 
@@ -98,7 +98,6 @@ class RomDataTracker:
         z_dest = quadratic(
             stage_position[axis] + (movement[axis] / pixel_step[axis]), *fit_params
         )
-
         return int(z_dest - stage_position["z"])
 
 
@@ -192,7 +191,7 @@ class RangeofMotionThing(lt.Thing):
 
         return {
             "Time": total_time,
-            "CSM Matrix": csm.image_to_stage_displacement_matrix.tolist(),
+            "CSM Matrix": csm.image_to_stage_displacement_matrix,
             "Step Range": step_range,
         }
 
diff --git a/tests/test_stage_measure.py b/tests/test_stage_measure.py
index ced2962b..85460972 100644
--- a/tests/test_stage_measure.py
+++ b/tests/test_stage_measure.py
@@ -44,12 +44,10 @@ def increasing_xyz_dict_generator(*_args, **_kwargs):
 @pytest.fixture
 def csm_matrix():
     """Return an example CSM matrix."""
-    return np.array(
-        [
-            [0.03061156624485296, 1.8031242270940833],
-            [1.773236372778601, 0.006660431608601435],
-        ]
-    )
+    return [
+        [0.03061156624485296, 1.8031242270940833],
+        [1.773236372778601, 0.006660431608601435],
+    ]
 
 
 @pytest.fixture