diff --git a/lisa/energy_model.py b/lisa/energy_model.py
index 6b65756e953b4680c9a2e96f10c1bb123d0c6247..208bb8812d5ea69b0fa4ac8295293e661b63af01 100644
--- a/lisa/energy_model.py
+++ b/lisa/energy_model.py
@@ -687,7 +687,7 @@ class EnergyModel(Serializable, Loggable):
         cpu_active_time = []
         for cpu, node in enumerate(self.cpu_nodes):
             assert (cpu,) == node.cpus
-            cap = node.active_states[freqs[cpu]].capacity
+            cap = node.active_states[int(freqs[cpu])].capacity
             cpu_active_time.append(min(float(cpu_utils[cpu]) / cap, 1.0))
 
         return self._estimate_from_active_time(cpu_active_time,