diff --git a/hopsflow/hopsflow.py b/hopsflow/hopsflow.py
index 7529b04..47588a3 100644
--- a/hopsflow/hopsflow.py
+++ b/hopsflow/hopsflow.py
@@ -370,7 +370,7 @@ def heat_flow_ensemble(
 def _interaction_energy_ensemble_body(
     ψs: Union[Tuple[np.ndarray, np.ndarray], Tuple[np.ndarray, np.ndarray, int]],
     params: SystemParams,
-    thermal: ThermalParams,
+    thermal: Optional[ThermalParams],
 ) -> np.ndarray:
     ψ_0, ψ_1 = ψs[0:2]
 
@@ -381,7 +381,7 @@ def _interaction_energy_ensemble_body(
     run = HOPSRun(ψ_0, ψ_1, params)
     energy = interaction_energy_coupling(run, params)
 
-    if isinstance(ys, int):
+    if thermal and (ys is not None):
         therm_run = ThermalRunParams(thermal, ys)
         energy += interaction_energy_therm(run, therm_run)