diff --git a/src/dopt_sensor_anomalies/detection.py b/src/dopt_sensor_anomalies/detection.py
index ac97668..57fbcce 100644
--- a/src/dopt_sensor_anomalies/detection.py
+++ b/src/dopt_sensor_anomalies/detection.py
@@ -166,7 +166,7 @@ def measure_length(
         # !! should only be newer OpenCV versions
         # box = cv2.cv.BoxPoints(rbox) if is_cv2() else cv2.boxPoints(rbox)
         box = cv2.boxPoints(rbox)
-        box = np.array(box, dtype="int")
+        box = np.array(box, dtype=np.int32)
         # order the points in the contour in top-left, top-right, bottom-right, and bottom-left
         box = perspective.order_points(box)