diff --git a/candlegp/training/hmc.py b/candlegp/training/hmc.py
index 747d189..9f1d325 100644
--- a/candlegp/training/hmc.py
+++ b/candlegp/training/hmc.py
@@ -50,7 +50,7 @@ def hmc_sample(model, num_samples, epsilon, lmin=1, lmax=2, thin=1, burn=0):
 
     def logprob_grads():
         logprob = -model.objective()
-        grads = torch.autograd.grad(logprob, model.parameters())
+        grads = torch.autograd.grad(logprob, [p for p in model.parameters() if p.requires_grad])
         return logprob, grads
 
     def thinning(thin_iterations, epsilon, lmin, lmax):