diff --git a/src/datachain/lib/dc.py b/src/datachain/lib/dc.py
index 11b26487a..53e400c60 100644
--- a/src/datachain/lib/dc.py
+++ b/src/datachain/lib/dc.py
@@ -128,14 +128,14 @@ def __init__(  # noqa: D107
         right_on: Optional[Sequence[Union[str, sqlalchemy.ColumnElement]]],
         msg: str,
     ):
-        def get_str(on: Sequence[Union[str, sqlalchemy.ColumnElement]]) -> str:
+        def _get_str(on: Sequence[Union[str, sqlalchemy.ColumnElement]]) -> str:
             if not isinstance(on, Sequence):
                 return str(on)  # type: ignore[unreachable]
             return ", ".join([_get_merge_error_str(col) for col in on])
 
-        on_str = get_str(on)
+        on_str = _get_str(on)
         right_on_str = (
-            ", right_on='" + get_str(right_on) + "'"
+            ", right_on='" + _get_str(right_on) + "'"
             if right_on and isinstance(right_on, Sequence)
             else ""
         )
@@ -283,8 +283,8 @@ def column(self, name: str) -> Column:
 
         raise ValueError(f"Column with name {name} not found in the schema")
 
-    def c(self, column: Union[str, Column]) -> "sqlalchemy.ColumnClause":
-        """Returns ColumnClause instance attached to the current chain."""
+    def c(self, column: Union[str, Column]) -> Column:
+        """Returns Column instance attached to the current chain."""
         c = self.column(column) if isinstance(column, str) else self.column(column.name)
         c.table = self.table
         return c
@@ -1205,7 +1205,7 @@ def merge(
 
         errors = []
 
-        def resolve(
+        def _resolve(
             ds: DataChain,
             col: Union[str, sqlalchemy.ColumnElement],
             side: Union[str, None],
@@ -1217,8 +1217,8 @@ def resolve(
                     errors.append(f"{_get_merge_error_str(col)} in {side}")
 
         ops = [
-            resolve(self, left, "left")
-            == resolve(right_ds, right, "right" if right_on else None)
+            _resolve(self, left, "left")
+            == _resolve(right_ds, right, "right" if right_on else None)
             for left, right in zip(on, right_on or on)
         ]
 
diff --git a/tests/unit/lib/test_datachain_merge.py b/tests/unit/lib/test_datachain_merge.py
index 1125c8b87..5612658ac 100644
--- a/tests/unit/lib/test_datachain_merge.py
+++ b/tests/unit/lib/test_datachain_merge.py
@@ -269,21 +269,27 @@ def test_merge_with_itself_column(test_session):
 
 
 def test_merge_on_expression(test_session):
-    ch = DataChain.from_values(team=team, session=test_session)
+    def _get_expr(dc):
+        c = dc.c("team.sport")
+        return func.substr(c, func.length(c) - 3)
+
+    dc = DataChain.from_values(team=team, session=test_session)
+    right_dc = dc.clone(new_table=True)
+
     # cross join on "ball" from sport
-    c = ch.c("team.sport")
-    expr = func.substr(c, func.length(c) - 4)
-    merged = ch.merge(ch, on=expr)
+    merged = dc.merge(right_dc, on=_get_expr(dc), right_on=_get_expr(right_dc))
 
-    cross_team = [(l_member, r_member) for l_member in team for r_member in team]
+    cross_team = [
+        (left_member, right_member) for left_member in team for right_member in team
+    ]
 
     count = 0
-    for left, right in merged.collect():
+    for left, right_dc in merged.collect():
         assert isinstance(left, TeamMember)
-        assert isinstance(right, TeamMember)
-        l_member, r_member = cross_team[count]
-        assert left == l_member
-        assert right == r_member
+        assert isinstance(right_dc, TeamMember)
+        left_member, right_member = cross_team[count]
+        assert left == left_member
+        assert right_dc == right_member
         count += 1
 
     assert count == len(team) * len(team)