diff --git a/dowhy/causal_estimators/two_stage_regression_estimator.py b/dowhy/causal_estimators/two_stage_regression_estimator.py
index 1ca1c8b67..a058571c4 100644
--- a/dowhy/causal_estimators/two_stage_regression_estimator.py
+++ b/dowhy/causal_estimators/two_stage_regression_estimator.py
@@ -116,17 +116,19 @@ def __init__(
         modified_target_estimand.identifier_method = "backdoor"
         modified_target_estimand.backdoor_variables = self._target_estimand.mediation_second_stage_confounders
         if second_stage_model is not None:
-            self._second_stage_model = (
-                second_stage_model
-                if isinstance(second_stage_model, CausalEstimator)
-                else second_stage_model(
+            if isinstance(second_stage_model, CausalEstimator):
+                self._second_stage_model = second_stage_model
+                # Update the estimand so the second-stage model uses the correct
+                # backdoor configuration rather than the original mediation estimand.
+                self._second_stage_model._target_estimand = modified_target_estimand
+            else:
+                self._second_stage_model = second_stage_model(
                     modified_target_estimand,
                     test_significance=self._significance_test,
                     evaluate_effect_strength=self._effect_strength_eval,
                     confidence_intervals=self._confidence_intervals,
                     **kwargs,
                 )
-            )
         else:
             self._second_stage_model = self.__class__.DEFAULT_SECOND_STAGE_MODEL(
                 modified_target_estimand,
diff --git a/tests/causal_estimators/test_two_stage_regression_estimator.py b/tests/causal_estimators/test_two_stage_regression_estimator.py
index c27b21c49..3174e9712 100644
--- a/tests/causal_estimators/test_two_stage_regression_estimator.py
+++ b/tests/causal_estimators/test_two_stage_regression_estimator.py
@@ -316,3 +316,52 @@ def test_nde_estimand_uses_correct_backdoor_variables(self):
         nde_estimand = estimator._second_stage_model_nde._target_estimand
         assert nde_estimand.identifier_method == "backdoor"
         assert nde_estimand.backdoor_variables == estimand.mediation_second_stage_confounders
+
+
+class TestTwoStageRegressionPreinstantiatedSecondStage:
+    """Regression tests for #1335: KeyError when second_stage_model is a pre-instantiated CausalEstimator.
+
+    When a user passes an already-constructed estimator instance as second_stage_model,
+    the TwoStageRegressionEstimator must update its _target_estimand to use the
+    modified (backdoor) estimand rather than the original mediation estimand.
+    """
+
+    def test_nie_with_preinstantiated_second_stage_no_keyerror(self):
+        """Passing a pre-instantiated second_stage_model must not raise KeyError."""
+        import statsmodels.api as sm
+
+        from dowhy.causal_estimators.generalized_linear_model_estimator import GeneralizedLinearModelEstimator
+
+        df = _make_mediation_data()
+        model = CausalModel(data=df, treatment="X", outcome="Y", graph=_MEDIATION_GML)
+        estimand = model.identify_effect(
+            estimand_type=EstimandType.NONPARAMETRIC_NIE,
+            proceed_when_unidentifiable=True,
+        )
+        second_stage = GeneralizedLinearModelEstimator(identified_estimand=estimand, glm_family=sm.families.Gaussian())
+        # This must not raise KeyError: None
+        estimate = model.estimate_effect(
+            identified_estimand=estimand,
+            method_name="mediation.two_stage_regression",
+            method_params={"second_stage_model": second_stage},
+        )
+        assert np.isfinite(estimate.value)
+
+    def test_nie_preinstantiated_second_stage_estimand_updated(self):
+        """The pre-instantiated second_stage_model's _target_estimand is updated to backdoor."""
+        import statsmodels.api as sm
+
+        from dowhy.causal_estimators.generalized_linear_model_estimator import GeneralizedLinearModelEstimator
+
+        df = _make_mediation_data()
+        model = CausalModel(data=df, treatment="X", outcome="Y", graph=_MEDIATION_GML)
+        estimand = model.identify_effect(
+            estimand_type=EstimandType.NONPARAMETRIC_NIE,
+            proceed_when_unidentifiable=True,
+        )
+        second_stage = GeneralizedLinearModelEstimator(identified_estimand=estimand, glm_family=sm.families.Gaussian())
+        estimator = TwoStageRegressionEstimator(
+            identified_estimand=estimand,
+            second_stage_model=second_stage,
+        )
+        assert estimator._second_stage_model._target_estimand.identifier_method == "backdoor"