diff --git a/quimb/experimental/belief_propagation/diis.py b/quimb/experimental/belief_propagation/diis.py
index 17502303..9544501e 100644
--- a/quimb/experimental/belief_propagation/diis.py
+++ b/quimb/experimental/belief_propagation/diis.py
@@ -75,7 +75,7 @@ def pack(self, tree):
         def extractor(x):
             if isinstance(x, Tensor):
                 x = x.data
-            arrays.append(self._reshape(x, -1))
+            arrays.append(self._reshape(x, (-1,)))
 
         arrays = []
         tree_apply(extractor, tree)