diff --git a/posteriors/vi/diag.py b/posteriors/vi/diag.py
index 7d5c883..7127f24 100644
--- a/posteriors/vi/diag.py
+++ b/posteriors/vi/diag.py
@@ -225,8 +225,9 @@ def nelbo(
 
     # Don't use vmap for single sample, since vmap doesn't work with lots of models
     if n_samples == 1:
-        log_p, aux = log_posterior(sampled_params[0], batch)
-        log_q = diag_normal_log_prob(sampled_params[0], mean, sd_diag)
+        single_param = tree_map(lambda x: x[0], sampled_params)
+        log_p, aux = log_posterior(single_param, batch)
+        log_q = diag_normal_log_prob(single_param, mean, sd_diag)
     else:
         log_p, aux = vmap(log_posterior, (0, None), (0, 0))(sampled_params, batch)
         log_q = vmap(diag_normal_log_prob, (0, None, None))(