diff --git a/src/qibo/backends/npmatrices.py b/src/qibo/backends/npmatrices.py
index 217697f006..490489833c 100644
--- a/src/qibo/backends/npmatrices.py
+++ b/src/qibo/backends/npmatrices.py
@@ -227,8 +227,8 @@ def GeneralizedfSim(self, u, phi):
         return self.np.array(
             [
                 [1, 0, 0, 0],
-                [0, u[0, 0], u[0, 1], 0],
-                [0, u[1, 0], u[1, 1], 0],
+                [0, complex(u[0, 0]), complex(u[0, 1]), 0],
+                [0, complex(u[1, 0]), complex(u[1, 1]), 0],
                 [0, 0, 0, phase],
             ],
             dtype=self.dtype,