Stored proc with execute as owner: error 916

I'm trying to use a stored proc with execute as owner to enable standard users to do something they can't normally do. But it's giving me error 916 when I try to execute it.

Here's the stored proc:

CREATE PROC dbo.Membership_DepartmentSync
    WITH EXECUTE AS owner
AS 
SET nocount ON;

SET IDENTITY_INSERT dbo.Membership_Department ON;

INSERT  dbo.Membership_Department
        (DepartmentId,
         Name)
        SELECT  DepartmentID,
                [Description]
        FROM    golfer.dbo.tblDepartment
        WHERE   IsDeleted = 0
                AND DepartmentID NOT IN (SELECT DepartmentID
                                         FROM   dbo.Membership_Department);

SET IDENTITY_INSERT dbo.Membership_Department OFF;

The idea is to copy in any Departments from the other database that we don't have in this one. When I execute it I get this error message:

Msg 916, Level 14, State 1, Procedure Membership_DepartmentSync, Line 10 The server principal "sa" is not able to access the database "golfer" under the current security context.

I'm stumped. sa is a member of the sysadmin role and I haven't denied it anything. sa owns both databases and is in the dbowner role in both databases. Both tables belong to the dbo schema. I'm logged in as myself and I'm a member of the sysadmin role.

I've tried a bit of diagnostics. I added some selects to see what 'owner' means at runtime and here are the results: suser_name() and suser_sname() return 'sa' while user_name() returns 'dbo'. That seems OK. I tried changing it to "with execute as 'sa'" but that gives me this error when I try to run the CREATE PROC:

Msg 15151, Level 16, State 1, Procedure Membership_DepartmentSync, Line 2 Cannot execute as the user 'sa', because it does not exist or you do not have permission.

This looks wrong. I'm a sysadmin.

The only things I'm finding about error 916 are strange things about SSMS bugs and unticking the Collation column in Object Explorer Details. I even tried that but it didn't change anything.

What am I not understanding? Please help!