diff --git a/mcbackend/backends/clickhouse.py b/mcbackend/backends/clickhouse.py
index ca4709e..31e5169 100644
--- a/mcbackend/backends/clickhouse.py
+++ b/mcbackend/backends/clickhouse.py
@@ -46,6 +46,10 @@
 }
 
 
+class ClickHouseBackendError(Exception):
+    """Something bad happened in the ClickHouse backend."""
+
+
 def create_runs_table(client: clickhouse_driver.Client):
     query = """
         CREATE TABLE IF NOT EXISTS runs (
@@ -80,7 +84,7 @@ def create_chain_table(client: clickhouse_driver.Client, meta: ChainMeta, rmeta:
     # Check that it does not already exist
     cid = chain_id(meta)
     if client.execute(f"SHOW TABLES LIKE '{cid}';"):
-        raise Exception(f"A table for {cid} already exists.")
+        raise ClickHouseBackendError(f"A table for {cid} already exists.")
 
     # Create a table with columns corresponding to the model variables
     columns = []
@@ -236,7 +240,7 @@ def _get_row_at(
         query = f"SELECT (`{names}`,) FROM {self.cid} WHERE _draw_idx={idx};"
         data = self._client.execute(query)
         if not data:
-            raise Exception(f"No record found for draw index {idx}.")
+            raise ClickHouseBackendError(f"No record found for draw index {idx}.")
         result = dict(zip(var_names, data[0][0]))
         return result
 
@@ -364,7 +368,10 @@ def __init__(
             raise ValueError("Either a `client` or a `client_fn` must be provided.")
 
         if client_fn is None:
-            client_fn = lambda: client
+
+            def client_fn():
+                return client
+
         if client is None:
             client = client_fn()
 
@@ -382,11 +389,11 @@ def init_run(self, meta: RunMeta) -> ClickHouseRun:
         else:
             created_at = datetime.now().astimezone(timezone.utc)
             query = "INSERT INTO runs (created_at, rid, proto) VALUES"
-            params = dict(
-                created_at=created_at,
-                rid=meta.rid,
-                proto=base64.encodebytes(bytes(meta)).decode("ascii"),
-            )
+            params = {
+                "created_at": created_at,
+                "rid": meta.rid,
+                "proto": base64.encodebytes(bytes(meta)).decode("ascii"),
+            }
             self._client.execute(query, [params])
         return ClickHouseRun(meta, client_fn=self._client_fn, created_at=created_at)
 
@@ -408,7 +415,9 @@ def get_run(self, rid: str) -> ClickHouseRun:
             {"rid": rid},
         )
         if len(rows) != 1:
-            raise Exception(f"Unexpected number of {len(rows)} results for rid='{rid}'.")
+            raise ClickHouseBackendError(
+                f"Unexpected number of {len(rows)} results for rid='{rid}'."
+            )
         data = base64.decodebytes(rows[0][2].encode("ascii"))
         meta = RunMeta().parse(data)
         return ClickHouseRun(
diff --git a/mcbackend/backends/numpy.py b/mcbackend/backends/numpy.py
index fb2b570..cb37f06 100644
--- a/mcbackend/backends/numpy.py
+++ b/mcbackend/backends/numpy.py
@@ -111,7 +111,7 @@ class NumPyRun(Run):
     """An MCMC run where samples are kept in memory."""
 
     def __init__(self, meta: RunMeta, *, preallocate: int) -> None:
-        self._settings = dict(preallocate=preallocate)
+        self._settings = {"preallocate": preallocate}
         self._chains: List[NumPyChain] = []
         super().__init__(meta)
 
@@ -129,9 +129,7 @@ class NumPyBackend(Backend):
     """An in-memory backend using NumPy."""
 
     def __init__(self, preallocate: int = 1_000) -> None:
-        self._settings = dict(
-            preallocate=preallocate,
-        )
+        self._settings = {"preallocate": preallocate}
         super().__init__()
 
     def init_run(self, meta: RunMeta) -> NumPyRun:
diff --git a/mcbackend/core.py b/mcbackend/core.py
index 02f28e3..6f627e7 100644
--- a/mcbackend/core.py
+++ b/mcbackend/core.py
@@ -27,6 +27,10 @@
 __all__ = ("is_rigid", "chain_id", "Chain", "Run", "Backend")
 
 
+class ChainError(Exception):
+    """Something is not right in one chain."""
+
+
 def is_rigid(nshape: Optional[Shape]):
     """Determines wheather the shape is constant.
 
@@ -119,7 +123,7 @@ def __len__(self) -> int:
         ]:
             for var in items:
                 return len(method(var.name))
-        raise Exception("This chain has no variables or sample stats.")
+        raise ChainError("This chain has no variables or sample stats.")
 
     @property
     def cid(self) -> str: