diff --git a/src/qibo/backends/numpy.py b/src/qibo/backends/numpy.py
index eba0e8204a..7248b2348e 100644
--- a/src/qibo/backends/numpy.py
+++ b/src/qibo/backends/numpy.py
@@ -400,7 +400,11 @@ def execute_circuit(self, circuit, initial_state=None, nshots=1000):
                 return self.execute_circuit(initial_state + circuit, None, nshots)
         elif initial_state is not None:
             initial_state = self.cast(initial_state)
-            valid_shape = 2 * (2**circuit.nqubits,) if circuit.density_matrix else (2**circuit.nqubits,)
+            valid_shape = (
+                2 * (2**circuit.nqubits,)
+                if circuit.density_matrix
+                else (2**circuit.nqubits,)
+            )
             if tuple(initial_state.shape) != valid_shape:
                 raise_error(
                     ValueError,