diff --git a/HearingTest.py b/HearingTest.py
index 0c664ea4c4aa864cd77fc3a64e02ccebc2c7914e..2c45908edf1bc71c552919ecb4bbcfe81babffbd 100644
--- a/HearingTest.py
+++ b/HearingTest.py
@@ -127,7 +127,7 @@ class HearingTest:
         return text
 
 
-def mean_two_arrays(a: np.ndarray, b: np.ndarray):
+def mean_two_arrays(a: np.ndarray, b: np.ndarray) -> np.ndarray:
     return np.mean(np.array([a, b]), axis=0)