diff --git a/kwant/lattice.py b/kwant/lattice.py
index 844815f888a0afe9a3edf0ba33599578146bea6b..78ab556c29b666fa1effa09c565b94774d7159d0 100644
--- a/kwant/lattice.py
+++ b/kwant/lattice.py
@@ -558,7 +558,7 @@ class TranslationalSymmetry(builder.Symmetry):
 
         det_m = int(round(np.linalg.det(m)))
         if det_m == 0:
-            raise ValueError('Singular symmetry matrix.')
+            raise RuntimeError('Adding site group failed.')
 
         det_x_inv_m = \
             np.array(np.round(det_m * np.linalg.inv(m)), dtype=int)