diff --git a/src/scanpy/preprocessing/_simple.py b/src/scanpy/preprocessing/_simple.py
index 1e3914460e..3cc5bcd804 100644
--- a/src/scanpy/preprocessing/_simple.py
+++ b/src/scanpy/preprocessing/_simple.py
@@ -649,7 +649,7 @@ def _create_regressor_categorical(
         # for each gene per category
         mask = category == cat_array
         for ix in numba.prange(X.T.shape[0]):
-            regressors[mask, ix] = X.T[ix][mask].mean()
+            regressors[mask, ix] = X.T[ix, mask].mean()
     return regressors