diff --git a/predtuner/modeledapp.py b/predtuner/modeledapp.py
index c0eeb47cbe64859550c3e348cd03d804e07c3171..a22c06dd6885551994afa3841eeca0b45ee6bdea 100644
--- a/predtuner/modeledapp.py
+++ b/predtuner/modeledapp.py
@@ -211,7 +211,8 @@ class QoSModelP1(IQoSModel):
         assert self.baseline_tensor is not None
         with_approxes = self.app.add_baseline_to_knobs(with_approxes)
         delta_tensors = np.array(
-            [self.delta_tensors[op][knob] for op, knob in with_approxes.items()]
+            [self.delta_tensors[op][knob] for op, knob in with_approxes.items()],
+            dtype=np.object
         )
         ret = delta_tensors.sum() + self.baseline_tensor
         return float(self.qos_metric(ret))