diff --git a/src/signature_mahalanobis_knn/mahal_distance.py b/src/signature_mahalanobis_knn/mahal_distance.py
index a1b9b46..d5939b9 100644
--- a/src/signature_mahalanobis_knn/mahal_distance.py
+++ b/src/signature_mahalanobis_knn/mahal_distance.py
@@ -23,6 +23,9 @@ def __init__(
         svd_thres : float, optional
             Threshold to decide numerical rank of the data matrix,
             by default 1e-12.
+        zero_thres : float, optional
+            Threshold to decide whether the distance is zero,
+            by default 1e-15.
         """
         self.subspace_thres: float = subspace_thres
         self.svd_thres: float = svd_thres