diff --git a/mitosis/__init__.py b/mitosis/__init__.py
index a8bf8bd..810ff47 100644
--- a/mitosis/__init__.py
+++ b/mitosis/__init__.py
@@ -157,7 +157,7 @@ def __init__(
         self.log_table = Table(table_name, md, *cols)
         url = "sqlite:///" + str(self.db)
         self.eng = create_engine(url)
-        with self.eng.connect() as conn:
+        with self.eng.begin() as conn:
             if not inspection.inspect(conn).has_table(table_name):
                 md.create_all(conn)
 
@@ -182,7 +182,7 @@ def emit(self, record: logging.LogRecord):
                         stmt = stmt.values({col: vals[i + 1]})
         else:
             raise ValueError("Cannot parse db message")
-        with self.eng.connect() as conn:
+        with self.eng.begin() as conn:
             conn.execute(stmt)
 
     def parse_record(self, msg: str) -> List[str]:
@@ -220,21 +220,21 @@ def _verify_variant_name(trial_db: Path, step: str, param: Parameter) -> None:
     eng = create_engine("sqlite:///" + str(trial_db))
     md = MetaData()
     tb = Table(f"{step}_variant_{param.arg_name}", md, *variant_types())
-    vals: Collection[Any]
+    vals: Collection
     if isinstance(param.vals, Mapping):
         vals = StrictlyReproduceableDict({k: v for k, v in sorted(param.vals.items())})
     elif isinstance(param.vals, Collection) and not isinstance(param.vals, str):
         try:
             vals = StrictlyReproduceableList(sorted(param.vals))
         except (ValueError, TypeError):
-            vals = param.vals
+            vals = str(param.vals)
     else:
-        vals = param.vals
+        vals = str(param.vals)
     df = pd.read_sql(select(tb), eng)
     ind_equal = df.loc[:, "name"] == param.var_name
     if ind_equal.sum() == 0:
         stmt = tb.insert().values({"name": param.var_name, "params": str(vals)})
-        with eng.connect() as conn:
+        with eng.begin() as conn:
             conn.execute(stmt)
     elif df.loc[ind_equal, "params"].iloc[0] != str(vals):
         raise RuntimeError(
diff --git a/mitosis/_typing.py b/mitosis/_typing.py
index 96c3609..4e0e224 100644
--- a/mitosis/_typing.py
+++ b/mitosis/_typing.py
@@ -1,23 +1,18 @@
-from abc import ABCMeta
+from collections.abc import Mapping
 from dataclasses import dataclass
 from dataclasses import field
-from types import ModuleType
-from typing import Any
 from typing import Callable
 from typing import NamedTuple
 from typing import ParamSpec
+from typing import TypedDict
 
 
-P = ParamSpec("P")
-ExpRun = Callable[P, dict]
+class ExpResults(TypedDict):
+    main: object
 
 
-class Experiment(ModuleType, metaclass=ABCMeta):
-    __name__: str
-    __file__: str
-    name: str
-    lookup_dict: dict[str, dict[str, Any]]
-    run: ExpRun
+P = ParamSpec("P")
+ExpRun = Callable[P, ExpResults]
 
 
 @dataclass
@@ -33,7 +28,7 @@ class Parameter:
 
     var_name: str
     arg_name: str
-    vals: Any
+    vals: object
     # > 3.10 only: https://stackoverflow.com/a/49911616/534674
     evaluate: bool = field(default=False, kw_only=True)
 
@@ -42,7 +37,7 @@ class ExpStep(NamedTuple):
     name: str
     action: ExpRun
     action_ref: str
-    lookup: dict[str, Any]
+    lookup: Mapping[str, Mapping[str, object]]
     lookup_ref: str
     group: str | None
     args: list[Parameter]
diff --git a/mitosis/tests/mock_paper.py b/mitosis/tests/mock_paper.py
index 0fe0493..8d9f975 100644
--- a/mitosis/tests/mock_paper.py
+++ b/mitosis/tests/mock_paper.py
@@ -1,3 +1,10 @@
+from collections import defaultdict
+
 data_config = {"length": {"test": 5}}
 
 meth_config = {"metric": {"test": "len"}}
+
+# lookup any parameter, any variant: always none
+lookup_default: dict[str, dict[str, None]] = defaultdict(
+    lambda: defaultdict(lambda: None)
+)
diff --git a/mitosis/tests/mock_part1.py b/mitosis/tests/mock_part1.py
index 916cb17..a8a5328 100644
--- a/mitosis/tests/mock_part1.py
+++ b/mitosis/tests/mock_part1.py
@@ -1,16 +1,23 @@
 from logging import getLogger
 
 import numpy as np
-from numpy.typing import NBitBase
-from numpy.typing import NDArray
+
+from mitosis._typing import ExpResults
 
 
 class Klass:
     @staticmethod
-    def gen_data(
-        length: int, extra: bool = False
-    ) -> dict[str, NDArray[np.floating[NBitBase]] | bool]:
+    def gen_data(length: int, extra: bool = False) -> ExpResults:
         getLogger(__name__).info("This is run every time")
         getLogger(__name__).debug("This is run in debug mode only")
 
-        return {"data": np.ones(length, dtype=np.float_), "extra": extra}
+        return {
+            "data": np.ones(length, dtype=np.float_),
+            "extra": extra,
+            "main": None,
+        }  # type: ignore
+
+
+def do_nothing(*args, **kwargs) -> ExpResults:
+    """An experiment step that accepts anything and produces nothing"""
+    return {"main": None}
diff --git a/mitosis/tests/mock_part2.py b/mitosis/tests/mock_part2.py
index 8cd3718..ff0d39e 100644
--- a/mitosis/tests/mock_part2.py
+++ b/mitosis/tests/mock_part2.py
@@ -5,15 +5,17 @@
 from numpy.typing import NBitBase
 from numpy.typing import NDArray
 
+from mitosis._typing import ExpResults
+
 
 def fit_and_score(
     data: NDArray[np.floating[NBitBase]], metric: Literal["len"] | Literal["zero"]
-) -> dict[str, float]:
+) -> ExpResults:
     if metric == "len":
         return {"main": len(data)}
     elif metric == "zero":
         return {"main": 0}
 
 
-def bad_runnable(*args: Any, **kwargs: Any):
+def bad_runnable(*args: Any, **kwargs: Any) -> int:
     return 1  # not a dict with key "main"
diff --git a/mitosis/tests/test_all.py b/mitosis/tests/test_all.py
index aa5bb2a..a000393 100644
--- a/mitosis/tests/test_all.py
+++ b/mitosis/tests/test_all.py
@@ -148,6 +148,33 @@ def test_mock_experiment(mock_steps, tmp_path):
     assert (metadata / "experiment").resolve().exists()
 
 
+@pytest.fixture
+def nothing_step():
+    # fmt: off
+    return ExpStep(
+        "nothing",
+        mock_part1.do_nothing, "mitosis.tests.mock_part1:do_nothing",
+        mock_paper.lookup_default, "mitosis.tests.mock_paper:lookup_default",
+        None,
+        [],
+        []
+    )
+    # fmt: on
+
+
+@pytest.mark.clean
+def test_variant_redefinition_disallowed(nothing_step, tmp_path):
+    # GH 56
+    chg_param1 = Parameter("foo_a", "foo", "a", evaluate=False)
+    chg_param2 = Parameter("foo_a", "foo", "b", evaluate=False)
+    nothing_step.args.append(chg_param1)
+    mitosis.run([nothing_step], trials_folder=tmp_path)
+    nothing_step.args.pop(0)
+    nothing_step.args.append(chg_param2)
+    with pytest.raises(RuntimeError, match="stored with different values"):
+        mitosis.run([nothing_step], trials_folder=tmp_path)
+
+
 def test_load_results_order(tmp_path):
     exp_key = "test_results"
     (tmp_path / exp_key).mkdir()
@@ -224,17 +251,11 @@ def test_malfored_return_experiment(mock_steps, tmp_path):
 def test_load_toml():
     parent = Path(__file__).resolve().parent
     tomlfile = parent / "test_pyproject.toml"
-    result = _disk.load_mitosis_steps(tomlfile)
-    expected = {
-        "data": (
-            "mitosis.tests.mock_part1:Klass.gen_data",
-            "mitosis.tests.mock_paper:data_config",
-        ),
-        "fit_eval": (
-            "mitosis.tests.mock_part2:fit_and_score",
-            "mitosis.tests.mock_paper:meth_config",
-        ),
-    }
+    result = _disk.load_mitosis_steps(tomlfile)["nothing"]
+    expected = (
+        "mitosis.tests.mock_part1:do_nothing",
+        "mitosis.tests.mock_paper:lookup_default",
+    )
     assert result == expected
 
 
diff --git a/mitosis/tests/test_pyproject.toml b/mitosis/tests/test_pyproject.toml
index 3cbb908..3544bde 100644
--- a/mitosis/tests/test_pyproject.toml
+++ b/mitosis/tests/test_pyproject.toml
@@ -1,4 +1,7 @@
 [tool.mitosis.steps]
+nothing = [
+    "mitosis.tests.mock_part1:do_nothing",
+    "mitosis.tests.mock_paper:lookup_default"]
 data = [
     "mitosis.tests.mock_part1:Klass.gen_data",
     "mitosis.tests.mock_paper:data_config"
diff --git a/pyproject.toml b/pyproject.toml
index 4f43008..517167f 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -43,7 +43,7 @@ dependencies = [
   "nbclient",
   "nbformat",
   "pandas<2.2",
-  "sqlalchemy",
+  "sqlalchemy>=1.4",
   "toml",
   "types-toml",
 ]