null_sentinels = {'text': r'\N'}

def ensure_not_null(db, value):

    if is_table_col(value):

        typed_col = db.col_info(value)

        if typed_col.nullable:

            try: null = null_sentinels[typed_col.type]

            except KeyError: pass

            else: return EnsureNotNull(value, null)

    return value