diff --git a/metalearners/metalearner.py b/metalearners/metalearner.py
index 6964547..c9539fd 100644
--- a/metalearners/metalearner.py
+++ b/metalearners/metalearner.py
@@ -227,8 +227,6 @@ def treatment_model_specifications(cls) -> dict[str, _ModelSpecifications]:
         """Return the specifications of all second-stage models."""
         ...
 
-    def _validate_params(self, **kwargs): ...
-
     @classmethod
     @abstractmethod
     def _supports_multi_treatment(cls) -> bool: ...
@@ -352,20 +350,6 @@ def __init__(
         n_folds: int | dict[str, int] = 10,
         random_state: int | None = None,
     ):
-        self._validate_params(
-            nuisance_model_factory=nuisance_model_factory,
-            treatment_model_factory=treatment_model_factory,
-            propensity_model_factory=propensity_model_factory,
-            is_classification=is_classification,
-            n_variants=n_variants,
-            nuisance_model_params=nuisance_model_params,
-            treatment_model_params=treatment_model_params,
-            propensity_model_params=propensity_model_params,
-            feature_set=feature_set,
-            n_folds=n_folds,
-            random_state=random_state,
-        )
-
         nuisance_model_specifications = self.nuisance_model_specifications()
         treatment_model_specifications = self.treatment_model_specifications()
 
diff --git a/metalearners/slearner.py b/metalearners/slearner.py
index 41b49a8..9d42522 100644
--- a/metalearners/slearner.py
+++ b/metalearners/slearner.py
@@ -8,13 +8,21 @@
 from sklearn.metrics import log_loss, root_mean_squared_error
 from typing_extensions import Self
 
-from metalearners._typing import Matrix, OosMethod, Vector
+from metalearners._typing import (
+    Features,
+    Matrix,
+    ModelFactory,
+    OosMethod,
+    Params,
+    Vector,
+    _ScikitModel,
+)
 from metalearners._utils import (
     convert_treatment,
     get_one,
     supports_categoricals,
 )
-from metalearners.cross_fit_estimator import OVERALL
+from metalearners.cross_fit_estimator import OVERALL, CrossFitEstimator
 from metalearners.metalearner import NUISANCE, MetaLearner, _ModelSpecifications
 
 _BASE_MODEL = "base_model"
@@ -88,7 +96,22 @@ def _supports_multi_treatment(cls) -> bool:
     def _supports_multi_class(cls) -> bool:
         return True
 
-    def _validate_params(self, feature_set, **kwargs):
+    def __init__(
+        self,
+        is_classification: bool,
+        n_variants: int,
+        nuisance_model_factory: ModelFactory | None = None,
+        treatment_model_factory: ModelFactory | None = None,
+        propensity_model_factory: type[_ScikitModel] | None = None,
+        nuisance_model_params: Params | dict[str, Params] | None = None,
+        treatment_model_params: Params | dict[str, Params] | None = None,
+        propensity_model_params: Params | None = None,
+        fitted_nuisance_models: dict[str, list[CrossFitEstimator]] | None = None,
+        fitted_propensity_model: CrossFitEstimator | None = None,
+        feature_set: Features | dict[str, Features] | None = None,
+        n_folds: int | dict[str, int] = 10,
+        random_state: int | None = None,
+    ):
         if feature_set is not None:
             # For SLearner it does not make sense to allow feature set as we only have one model
             # and having it would bring problems when using fit_nuisance and predict_nuisance
@@ -97,6 +120,21 @@ def _validate_params(self, feature_set, **kwargs):
                 "Base-model specific feature_sets were provided to S-Learner. "
                 "These will be ignored and all available features will be used instead."
             )
+        super().__init__(
+            is_classification=is_classification,
+            n_variants=n_variants,
+            nuisance_model_factory=nuisance_model_factory,
+            treatment_model_factory=treatment_model_factory,
+            propensity_model_factory=propensity_model_factory,
+            nuisance_model_params=nuisance_model_params,
+            treatment_model_params=treatment_model_params,
+            propensity_model_params=propensity_model_params,
+            fitted_nuisance_models=fitted_nuisance_models,
+            fitted_propensity_model=fitted_propensity_model,
+            feature_set=None,
+            n_folds=n_folds,
+            random_state=random_state,
+        )
 
     def fit(
         self,
diff --git a/tests/test_slearner.py b/tests/test_slearner.py
index 729f20b..37f6086 100644
--- a/tests/test_slearner.py
+++ b/tests/test_slearner.py
@@ -10,12 +10,21 @@
 from metalearners.slearner import SLearner, _append_treatment_to_covariates
 
 
-def test_feature_set_doesnt_raise():
-    SLearner(
+def test_feature_set_doesnt_raise(rng):
+    slearner = SLearner(
         nuisance_model_factory=LinearRegression,
         is_classification=False,
         n_variants=2,
-        feature_set="",
+        feature_set=[0],
+    )
+
+    X = rng.standard_normal((100, 2))
+    y = rng.standard_normal(100)
+    w = rng.integers(0, 2, 100)
+    slearner.fit(X, y, w)
+    assert (
+        slearner._nuisance_models["base_model"][0]._overall_estimator.n_features_in_  # type: ignore
+        == 3
     )