diff --git a/gridlock/read.py b/gridlock/read.py
index 28afdd2..44f1c8a 100644
--- a/gridlock/read.py
+++ b/gridlock/read.py
@@ -87,7 +87,7 @@ class GridReadMixin(GridPosMixin):
             sample_period: int = 1,
             finalize: bool = True,
             pcolormesh_args: dict[str, Any] | None = None,
-            ax: 'matplotlib.axes.Axes' | None = None,
+            ax: 'matplotlib.axes.Axes | None' = None,
             ) -> tuple['matplotlib.figure.Figure', 'matplotlib.axes.Axes']:
         """
         Visualize a slice of a grid.
@@ -149,7 +149,7 @@ class GridReadMixin(GridPosMixin):
             which_shifts: int = 0,
             finalize: bool = True,
             contour_args: dict[str, Any] | None = None,
-            ax: 'matplotlib.axes.Axes' | None = None,
+            ax: 'matplotlib.axes.Axes | None' = None,
             level_fraction: float = 0.7,
             ) -> tuple['matplotlib.figure.Figure', 'matplotlib.axes.Axes']:
         """