diff --git a/mp_api/client/routes/materials/grain_boundaries.py b/mp_api/client/routes/materials/grain_boundaries.py
index 1c73ac970..1721ccd64 100644
--- a/mp_api/client/routes/materials/grain_boundaries.py
+++ b/mp_api/client/routes/materials/grain_boundaries.py
@@ -20,7 +20,7 @@ def search(
         gb_plane: list[str] | None = None,
         gb_energy: tuple[float, float] | None = None,
         pretty_formula: str | None = None,
-        rotation_axis: list[str] | None = None,
+        rotation_axis: tuple[int, int, int] | tuple[int, int, int, int] | None = None,
         rotation_angle: tuple[float, float] | None = None,
         separation_energy: tuple[float, float] | None = None,
         sigma: int | None = None,
@@ -40,8 +40,10 @@ def search(
              material_ids (List[str]): List of Materials Project IDs to query with.
              pretty_formula (str): Formula of the material.
              rotation_angle (Tuple[float,float]): Minimum and maximum rotation angle in degrees to consider.
-             rotation_axis(List[str]): The Miller index of rotation axis. e.g., [1, 0, 0], [1, 1, 0], and [1, 1, 1]
-                 sigma (int): Sigma value of grain boundary.
+             rotation_axis (tuple of 3 int or of 4 int): The Miller index of rotation axis.
+                A 3- or 4-tuple of int or str: e.g.,
+                (0, 0, 0, 1), (1, 0, 0), (1, 1, 0), or (1, 1, 1)
+             sigma (int): Sigma value of grain boundary.
              separation_energy (Tuple[float,float]): Minimum and maximum work of separation energy in J/m³ to consider.
              sigma (int): Sigma value of the boundary.
              type (GBTypeEnum): Grain boundary type.
@@ -84,6 +86,11 @@ def search(
             )
 
         if rotation_axis:
+            if len(rotation_axis) not in {3, 4}:
+                raise ValueError(
+                    "`rotation_axis` should be a tuple of either "
+                    "3 or 4 int values, ex: (0, 0, 0, 1) or (1, 0, 0)"
+                )
             query_params.update(
                 {"rotation_axis": ",".join([str(n) for n in rotation_axis])}
             )