diff --git a/src/datachain/lib/convert/values_to_tuples.py b/src/datachain/lib/convert/values_to_tuples.py
index 9f0b4cc07..9675a6c11 100644
--- a/src/datachain/lib/convert/values_to_tuples.py
+++ b/src/datachain/lib/convert/values_to_tuples.py
@@ -1,7 +1,12 @@
 from collections.abc import Sequence
 from typing import Any, Union
 
-from datachain.lib.data_model import DataType, DataTypeNames, is_chain_type
+from datachain.lib.data_model import (
+    DataType,
+    DataTypeNames,
+    DataValuesType,
+    is_chain_type,
+)
 from datachain.lib.utils import DataChainParamsError
 
 
@@ -15,7 +20,7 @@ def __init__(self, ds_name, msg):
 def values_to_tuples(  # noqa: C901, PLR0912
     ds_name: str = "",
     output: Union[None, DataType, Sequence[str], dict[str, DataType]] = None,
-    **fr_map,
+    **fr_map: Sequence[DataValuesType],
 ) -> tuple[Any, Any, Any]:
     if output:
         if not isinstance(output, (Sequence, str, dict)):
@@ -47,10 +52,10 @@ def values_to_tuples(  # noqa: C901, PLR0912
                 f" number of signals '{len(fr_map)}'",
             )
 
-    types_map = {}
+    types_map: dict[str, type] = {}
     length = -1
     for k, v in fr_map.items():
-        if not isinstance(v, Sequence) or isinstance(v, str):
+        if not isinstance(v, Sequence) or isinstance(v, str):  # type: ignore[unreachable]
             raise ValuesToTupleError(ds_name, f"signals '{k}' is not a sequence")
         len_ = len(v)
 
@@ -64,15 +69,16 @@ def values_to_tuples(  # noqa: C901, PLR0912
             if len_ == 0:
                 raise ValuesToTupleError(ds_name, f"signal '{k}' is empty list")
 
-            typ = type(v[0])
+            first_element = next(iter(v))
+            typ = type(first_element)
             if not is_chain_type(typ):
                 raise ValuesToTupleError(
                     ds_name,
                     f"signal '{k}' has unsupported type '{typ.__name__}'."
                     f" Please use DataModel types: {DataTypeNames}",
                 )
-            if typ is list:
-                types_map[k] = list[type(v[0][0])]  # type: ignore[misc]
+            if isinstance(first_element, list):
+                types_map[k] = list[type(first_element[0])]  # type: ignore[assignment, misc]
             else:
                 types_map[k] = typ
 
@@ -98,7 +104,7 @@ def values_to_tuples(  # noqa: C901, PLR0912
     if len(output) > 1:  # type: ignore[arg-type]
         tuple_type = tuple(output_types)
         res_type = tuple[tuple_type]  # type: ignore[valid-type]
-        res_values = list(zip(*fr_map.values()))
+        res_values: Sequence[Any] = list(zip(*fr_map.values()))
     else:
         res_type = output_types[0]  # type: ignore[misc]
         res_values = next(iter(fr_map.values()))
diff --git a/src/datachain/lib/data_model.py b/src/datachain/lib/data_model.py
index 5afb0ac63..bcb58b915 100644
--- a/src/datachain/lib/data_model.py
+++ b/src/datachain/lib/data_model.py
@@ -18,6 +18,7 @@
 ]
 DataType = Union[type[BaseModel], StandardType]
 DataTypeNames = "BaseModel, int, str, float, bool, list, dict, bytes, datetime"
+DataValuesType = Union[BaseModel, int, str, float, bool, list, dict, bytes, datetime]
 
 
 class DataModel(BaseModel):