diff --git a/midas/checker/frame_methods.py b/midas/checker/frame_methods.py
index 2afadfd..c2935f8 100644
--- a/midas/checker/frame_methods.py
+++ b/midas/checker/frame_methods.py
@@ -40,6 +40,7 @@ def frame_method(*names: str):
 class Call:
     location: Location
     frame: DataFrameType
+    frame_expr: p.Expr
     positional: list[TypedExpr]
     keywords: dict[str, TypedExpr]
 
@@ -105,6 +106,7 @@ class MethodRegistry(metaclass=_MethodRegistryMeta):
 
         by_name: dict[str, DataFrameType.Column] = {}
         frame2: Optional[DataFrameType] = None
+        # Get map of operand's columns by name, if there is at least 1 operand, which is a dataframe
         if len(call.positional) != 0:
             other: Type = call.positional[0][1]
             unfolded_other: Type = unfold_type(other)
@@ -114,6 +116,10 @@ class MethodRegistry(metaclass=_MethodRegistryMeta):
                     col.name: col for col in frame2.columns if col.name is not None
                 }
 
+        # Compute new schema:
+        # Step 1: for all columns in frame1:
+        #  - if present in frame2 with equivalent type -> add to schema as is
+        #  - if not -> add to schema as unknown
         in_frame1: set[str] = set()
         for column in call.frame.columns:
             if column.name is not None:
@@ -134,6 +140,8 @@ class MethodRegistry(metaclass=_MethodRegistryMeta):
             )
             new_columns.append(new_column)
 
+        # Step 2: for all columns in frame2
+        #  - if not in frame1 -> add to schema as unknown
         if frame2 is not None:
             for column in frame2.columns:
                 if column.name in in_frame1:
@@ -146,6 +154,7 @@ class MethodRegistry(metaclass=_MethodRegistryMeta):
                     )
                 )
 
+        # Build signature with new schema and generic operand
         signature = Function(
             args=[
                 Function.Argument(
@@ -158,6 +167,7 @@ class MethodRegistry(metaclass=_MethodRegistryMeta):
             returns=DataFrameType(columns=new_columns),
         )
 
+        # Map arguments and compute result type
         result: CallResult = self.dispatcher.get_result(
             location=call.location,
             callee=signature,
diff --git a/midas/checker/frames.py b/midas/checker/frames.py
index da9eb0e..56c34c6 100644
--- a/midas/checker/frames.py
+++ b/midas/checker/frames.py
@@ -142,12 +142,14 @@ class FrameManager:
         method: str,
         location: Location,
         frame: DataFrameType,
+        frame_expr: p.Expr,
         positional: list[TypedExpr],
         keywords: dict[str, TypedExpr],
     ) -> Type:
         call: Call = Call(
             location=location,
             frame=frame,
+            frame_expr=frame_expr,
             positional=positional,
             keywords=keywords,
         )
diff --git a/midas/checker/python.py b/midas/checker/python.py
index 70cb4c9..374f736 100644
--- a/midas/checker/python.py
+++ b/midas/checker/python.py
@@ -211,23 +211,24 @@ class PythonTyper(
     def call_method(
         self,
         location: Location,
-        obj: Type,
+        obj: TypedExpr,
         method_name: str,
         positional: list[TypedExpr],
         keywords: dict[str, TypedExpr],
     ) -> Optional[Type]:
-        unfolded: Type = unfold_type(obj)
+        unfolded: Type = unfold_type(obj[1])
         match unfolded:
             case DataFrameType():
                 return self.frame_mgr.call(
                     method=method_name,
                     location=location,
                     frame=unfolded,
+                    frame_expr=obj[0],
                     positional=positional,
                     keywords=keywords,
                 )
 
-        method: Optional[Type] = self.types.lookup_member(obj, method_name)
+        method: Optional[Type] = self.types.lookup_member(obj[1], method_name)
         if method is None:
             raise UndefinedMethodException
 
@@ -522,7 +523,9 @@ class PythonTyper(
 
         result: Optional[Type]
         try:
-            result = self.call_method(location, left, method, [(right_expr, right)], {})
+            result = self.call_method(
+                location, (left_expr, left), method, [(right_expr, right)], {}
+            )
         except UndefinedMethodException:
             self.reporter.error(
                 location,
@@ -545,7 +548,9 @@ class PythonTyper(
 
         result: Optional[Type]
         try:
-            result = self.call_method(expr.location, operand, method, [], {})
+            result = self.call_method(
+                expr.location, (expr.right, operand), method, [], {}
+            )
         except UndefinedMethodException:
             self.reporter.error(
                 expr.location,
@@ -573,11 +578,12 @@ class PythonTyper(
                 unfolded: Type = unfold_type(obj_type)
                 if isinstance(unfolded, DataFrameType):
                     return self.frame_mgr.call(
-                        method,
-                        expr.location,
-                        unfolded,
-                        positional,
-                        keywords,
+                        method=method,
+                        location=expr.location,
+                        frame=unfolded,
+                        frame_expr=obj,
+                        positional=positional,
+                        keywords=keywords,
                     )
 
         callee: Type = self.type_of(expr.callee)