sql - 代替スキーマでPL/SQLを実行

Question

UserA としてログインし、次を実行しようとしていました

alter session set current_schema = UserB;
begin
    execute immediate 'Insert into UserB.tablea (c1, c2) values (val1,val2)'
end;

このエラーが発生し続けます

ORA-01031: insufficient privileges
ORA-06512: at line 3
01031. 00000 -  "insufficient privileges"
*Cause:    An attempt was made to change the current username or password
       without the appropriate privilege. This error also occurs if
       attempting to install a database without the necessary operating
       system privileges.
       When Trusted Oracle is configure in DBMS MAC, this error may occur
       if the user was granted the necessary privilege at a higher label
       than the current login.
*Action:   Ask the database administrator to perform the operation or grant
       the required privileges.
       For Trusted Oracle users getting this error although granted the
       the appropriate privilege at a higher label, ask the database
       administrator to regrant the privilege at the appropriate label.

UserA に alter_session 権限があり、UserB.tablea に UserA として権限を挿入します。何か案は？