diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d10ea33..d88a0bf 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -20,6 +20,18 @@ jobs:
       - name: Run tests with coverage
         run: uv run --all-extras pytest --cov=centimators --cov-report=term
 
+  # The lockfile tracks latest narwhals; the declared floor is much older and
+  # narwhals 1.x exposes typing aliases as runtime strings, which annotations
+  # can trip on at import time. Smoke-test imports at the floor so a green
+  # lockfile can't hide a broken install for downstream pins.
+  test-narwhals-floor:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: astral-sh/setup-uv@v4
+      - name: Import smoke test on minimum supported narwhals
+        run: uv run --all-extras --with "narwhals==1.39.0" pytest tests/test_imports.py
+
   lint:
     runs-on: ubuntu-latest
     steps:
diff --git a/pyproject.toml b/pyproject.toml
index a6eb10f..84a0ab2 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [project]
 name = "centimators"
-version = "0.3.4"
+version = "0.3.5"
 description = "essential data transformers and model estimators for ML and data science competitions"
 readme = "README.md"
 authors = [
diff --git a/src/centimators/feature_transformers/neutralization.py b/src/centimators/feature_transformers/neutralization.py
index a2c47ba..640f31d 100644
--- a/src/centimators/feature_transformers/neutralization.py
+++ b/src/centimators/feature_transformers/neutralization.py
@@ -1,5 +1,7 @@
 """Neutralization transformers for reducing feature exposure."""
 
+from __future__ import annotations
+
 import warnings
 
 import narwhals as nw
diff --git a/src/centimators/feature_transformers/penalization.py b/src/centimators/feature_transformers/penalization.py
index 68e36de..374f7f2 100644
--- a/src/centimators/feature_transformers/penalization.py
+++ b/src/centimators/feature_transformers/penalization.py
@@ -1,5 +1,7 @@
 """Feature penalization transformers using iterative optimization (requires JAX)."""
 
+from __future__ import annotations
+
 import warnings
 
 import narwhals as nw
diff --git a/tests/test_imports.py b/tests/test_imports.py
new file mode 100644
index 0000000..86fbb0f
--- /dev/null
+++ b/tests/test_imports.py
@@ -0,0 +1,29 @@
+"""Import smoke tests.
+
+Every module must import cleanly (or fail only with ImportError for optional
+dependencies) on every supported narwhals version. Regression guard for the
+narwhals 1.x incident where ``IntoSeries | None`` in a runtime-evaluated
+signature raised TypeError at import time: narwhals 1.x exposes
+``narwhals.typing`` aliases as plain strings at runtime, so any module using
+them in annotations must carry ``from __future__ import annotations``.
+"""
+
+import importlib
+import pkgutil
+
+import centimators
+
+
+def test_top_level_import():
+    assert hasattr(centimators, "RankTransformer")
+
+
+def test_all_modules_import():
+    prefix = centimators.__name__ + "."
+    for mod_info in pkgutil.walk_packages(centimators.__path__, prefix):
+        try:
+            importlib.import_module(mod_info.name)
+        except ImportError:
+            # Optional dependencies (jax, dspy, keras, umap) may be absent;
+            # anything else (e.g. TypeError from annotation evaluation) fails.
+            continue
diff --git a/uv.lock b/uv.lock
index 6ea235d..94526e6 100644
--- a/uv.lock
+++ b/uv.lock
@@ -371,7 +371,7 @@ wheels = [
 
 [[package]]
 name = "centimators"
-version = "0.3.4"
+version = "0.3.5"
 source = { editable = "." }
 dependencies = [
     { name = "narwhals" },