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bigframes/core/blocks.py

@@ -1039,7 +1039,7 @@ def aggregate_all_and_stack(
             ]
             # TODO: may need add NullaryAggregation in main_aggregation
             # when agg add support for axis=1, needed for agg("size", axis=1)
-            assert isinstance(operation, agg_ops.UnaryAggregateOp)
+            assert isinstance(operation, agg_ops.UnaryAggregateOp), f"Expected a unary operation, but got {operation}. Please report this error and how you got here to the BigQuery DataFrames team (bit.ly/bigframes-feedback)."
             main_aggregation = (
                 ex.UnaryAggregation(operation, ex.free_var(value_col_id)),
                 value_col_id,

bigframes/operations/aggregations.py

@@ -487,7 +487,6 @@ def output_type(self, *input_types: dtypes.ExpressionType) -> dtypes.ExpressionT
 
 
 # TODO: Alternative names and lookup from numpy function objects
-# Add size_op separately to avoid Mypy type inference errors.
 _AGGREGATIONS_LOOKUP: typing.Dict[
     str, typing.Union[UnaryAggregateOp, NullaryAggregateOp]
 ] = {
@@ -509,7 +508,10 @@ def output_type(self, *input_types: dtypes.ExpressionType) -> dtypes.ExpressionT
         ApproxQuartilesOp(2),
         ApproxQuartilesOp(3),
     ]
-    + [size_op]
+    + [
+        # Add size_op separately to avoid Mypy type inference errors.
+        size_op,
+    ]
 }