diff --git a/src/qibo/backends/numpy.py b/src/qibo/backends/numpy.py
index cc90b75862..15c5d7cd2f 100644
--- a/src/qibo/backends/numpy.py
+++ b/src/qibo/backends/numpy.py
@@ -750,7 +750,7 @@ def calculate_matrix_exp(self, a, matrix, eigenvectors=None, eigenvalues=None):
         return self.np.matmul(eigenvectors, self.np.matmul(expd, ud))
 
     def calculate_expectation_state(self, hamiltonian, state, normalize):
-        statec = np.conj(state)
+        statec = self.np.conj(state)
         hstate = hamiltonian @ state
         ev = self.np.real(self.np.sum(statec * hstate))
         if normalize: