diff --git a/nifty5/operators/diagonal_operator.py b/nifty5/operators/diagonal_operator.py
index ae148448d2e58b64a0f64d3993b4eb0c1e63130c..1cce2068bf381528cf504cbd98674f69084947de 100644
--- a/nifty5/operators/diagonal_operator.py
+++ b/nifty5/operators/diagonal_operator.py
@@ -105,7 +105,7 @@ class DiagonalOperator(EndomorphicOperator):
             res._spaces = None
         else:
             res._spaces = tuple(set(self._spaces) | set(spc))
-        res._ldiag = ldiag
+        res._ldiag = np.array(ldiag)
         res._fill_rest()
         return res