diff --git a/regional_mom6/regional_mom6.py b/regional_mom6/regional_mom6.py
index 98e30a19..2cb4d47b 100644
--- a/regional_mom6/regional_mom6.py
+++ b/regional_mom6/regional_mom6.py
@@ -410,6 +410,9 @@ def rectangular_hgrid(λ, φ):
         xarray.Dataset: An FMS-compatible ``hgrid`` that includes all required attributes.
     """
 
+    assert np.all(np.diff(λ) > 0), "longitudes array λ must be monotonically increasing"
+    assert np.all(np.diff(φ) > 0), "latitudes array φ must be monotonically increasing"
+
     R = 6371e3  # mean radius of the Earth; https://en.wikipedia.org/wiki/Earth_radius
 
     # compute longitude spacing and ensure that longitudes are uniformly spaced