diff --git a/toqito/perms/permute_systems.py b/toqito/perms/permute_systems.py
index 0da495345..2bb5c66c3 100644
--- a/toqito/perms/permute_systems.py
+++ b/toqito/perms/permute_systems.py
@@ -210,8 +210,10 @@ def permute_systems(
     row_perm = permute_systems(vec_arg, perm, dim[0][:], False, inv_perm)
 
     # This condition is only necessary if the `input_mat` variable is sparse.
-    if isinstance(input_mat, (sparse.csr_array, sparse.dia_array)):
+    if isinstance(input_mat, sparse.csr_array):
         input_mat = input_mat.toarray()
+    elif isinstance(input_mat, sparse.dia_matrix):
+        input_mat = input_mat.tocsr()
         permuted_mat = input_mat[row_perm, :]
         permuted_mat = np.array(permuted_mat)
     else: