diff --git a/src/operators/diagonal_operator.py b/src/operators/diagonal_operator.py
index f40696e43bac705e6882e72efc81d0c524c1bd08..297eba7a8714d89d304cdd6f21a701da566eed9d 100644
--- a/src/operators/diagonal_operator.py
+++ b/src/operators/diagonal_operator.py
@@ -72,7 +72,7 @@ class DiagonalOperator(EndomorphicOperator):
 
     def __init__(self, diagonal, domain=None, spaces=None, sampling_dtype=None,
                  _trafo=0):
-# MR: _trafo is more or less deliberately undocumened, since it is not supposed
+# MR: _trafo is more or less deliberately undocumented, since it is not supposed
 # to be necessary for "end users". It describes the type of transform for which
 # the diagonal can be used without modification
 # (0:TIMES, 1:ADJOINT, 2:INVERSE, 3:ADJOINT_INVERSE)