diff --git a/src/gflownet/models/seq_transformer.py b/src/gflownet/models/seq_transformer.py
index e0673430..da9e52d9 100644
--- a/src/gflownet/models/seq_transformer.py
+++ b/src/gflownet/models/seq_transformer.py
@@ -60,6 +60,8 @@ def __init__(
         else:
             self.output = MLPWithDropout(num_hid, num_outs, [2 * num_hid, 2 * num_hid], mc.dropout)
         self.num_hid = num_hid
+        # TODO: Merge non-autoregressive implementations of sequence generation
+        assert not cfg.model.do_separate_pb, "Not implemented for SeqTransformerGFN (since P_B=1 when autoregressive)."
 
     def forward(self, xs: SeqBatch, cond, batched=False):
         """Returns a GraphActionCategorical and a tensor of state predictions.