diff --git a/tools/infer/utility.py b/tools/infer/utility.py
index 3fa62c775c53a9bb48c19d66f79430859d42d3f3..613e42ef76f1acb2bd7223bdc8c77274cc842aab 100755
--- a/tools/infer/utility.py
+++ b/tools/infer/utility.py
@@ -625,6 +625,7 @@ def get_rotate_crop_image(img, points):
     points[:, 0] = points[:, 0] - left
     points[:, 1] = points[:, 1] - top
     '''
+    assert len(points) == 4, "shape of points must be 4*2"
     img_crop_width = int(
         max(
             np.linalg.norm(points[0] - points[1]),