diff --git a/zuko/flows.py b/zuko/flows.py
index ff78042..1566adf 100644
--- a/zuko/flows.py
+++ b/zuko/flows.py
@@ -869,7 +869,7 @@ def forward(self, y: Tensor = None) -> Transform:
         return FreeFormJacobianTransform(
             f=partial(self.f, y=y),
             time=self.time,
-            phi=(y, *self.ode.parameters()),
+            phi=self.ode.parameters() if y is None else (y, *self.ode.parameters()),
             exact=self.exact,
         )