diff --git a/src/qibojit/backends/cpu.py b/src/qibojit/backends/cpu.py
index 4ae8dea..ce823ba 100644
--- a/src/qibojit/backends/cpu.py
+++ b/src/qibojit/backends/cpu.py
@@ -152,14 +152,21 @@ def _create_qubits_tensor(gate, nqubits):
 
     def _as_custom_matrix(self, gate):
         name = gate.__class__.__name__
+        _matrix = getattr(self.custom_matrices, name)
+
         if isinstance(gate, ParametrizedGate):
-            return getattr(self.custom_matrices, name)(*gate.parameters)
-        elif isinstance(gate, FusedGate):  # pragma: no cover
+            if name == "GeneralizedRBS":  # pragma: no cover
+                # this is tested in qibo tests
+                theta = gate.init_kwargs["theta"]
+                phi = gate.init_kwargs["phi"]
+                return _matrix(gate.init_args[0], gate.init_args[1], theta, phi)
+            return _matrix(*gate.parameters)
+
+        if isinstance(gate, FusedGate):  # pragma: no cover
             # fusion is tested in qibo tests
             return self.matrix_fused(gate)
-        else:
-            matrix = getattr(self.custom_matrices, name)
-            return matrix(2 ** len(gate.target_qubits)) if callable(matrix) else matrix
+
+        return _matrix(2 ** len(gate.target_qubits)) if callable(_matrix) else _matrix
 
     def apply_gate(self, gate, state, nqubits):
         matrix = self._as_custom_matrix(gate)