diff --git a/pygmt/datasets/mars_relief.py b/pygmt/datasets/mars_relief.py
index a7059c1e2e5..01111ccc871 100644
--- a/pygmt/datasets/mars_relief.py
+++ b/pygmt/datasets/mars_relief.py
@@ -62,7 +62,7 @@ def load_mars_relief(
         [*xmin*, *xmax*, *ymin*, *ymax*] or a string *xmin/xmax/ymin/ymax*.
         Required for grids with resolutions higher than 5 arc-minutes
         (i.e., ``"05m"``).
-    registration : str
+    registration
         Grid registration type. Either ``"pixel"`` for pixel registration or
         ``"gridline"`` for gridline registration. Default is ``None``, means
         ``"gridline"`` for all resolutions except for ``"12s"`` which is