Why can't I perform a simple count in a subquery?

I'm trying to write a simple query that fetches all the columns of a specific table, but also adds a count of rows from a related table.

Here is the query:

SELECT 
   todos.*, 
   (SELECT COUNT(assets.*) FROM assets WHERE assets.parent_id = todos._id) AS asset_count 
FROM todos

Here is the error:

Caused by: android.database.sqlite.SQLiteException: near "*": syntax error (code 1): , while compiling: SELECT todos.*, (SELECT COUNT(assets.*) FROM assets WHERE assets.parent_id = todos._id) AS asset_count FROM todos