diff --git a/openwfs/algorithms/dual_reference.py b/openwfs/algorithms/dual_reference.py
index 03bda5b..b81f828 100644
--- a/openwfs/algorithms/dual_reference.py
+++ b/openwfs/algorithms/dual_reference.py
@@ -1,4 +1,4 @@
-from typing import Optional
+from typing import Optional, Union
 
 import numpy as np
 from numpy import ndarray as nd
@@ -38,7 +38,7 @@ def __init__(
         feedback: Detector,
         slm: PhaseSLM,
         phase_patterns: Optional[tuple[nd, nd]],
-        amplitude: Optional[tuple[nd, nd] | str],
+        amplitude: Optional[Union[tuple[nd, nd], str]],
         group_mask: nd,
         phase_steps: int = 4,
         iterations: int = 2,
diff --git a/openwfs/plot_utilities.py b/openwfs/plot_utilities.py
index 7391aa9..33a76bf 100644
--- a/openwfs/plot_utilities.py
+++ b/openwfs/plot_utilities.py
@@ -1,4 +1,4 @@
-from typing import Tuple
+from typing import Tuple, Union
 
 import numpy as np
 from numpy import ndarray as nd
@@ -56,7 +56,7 @@ def scale_prefix(value: u.Quantity) -> u.Quantity:
         return value
 
 
-def slope_step(a: nd, width: nd | float) -> nd:
+def slope_step(a: nd, width: Union[nd, float]) -> nd:
     """
     A sloped step function from 0 to 1.