diff --git a/ortools/sat/python/cp_model.py b/ortools/sat/python/cp_model.py
index 6ea1fa3bb6..7998e9ec59 100644
--- a/ortools/sat/python/cp_model.py
+++ b/ortools/sat/python/cp_model.py
@@ -1601,7 +1601,7 @@ class CpSolver(object):
 
     def __init__(self):
         self.__model = None
-        self.__solution = None
+        self.__solution: cp_model_pb2.CpSolverResponse = None
         self.parameters = sat_parameters_pb2.SatParameters()
 
     def Solve(self, model):