Automate adding security to a pdf while saving

I have seen several people trying to do a similar thing in this forum, but I didn't see an actual working solution...

I have a livecycle pdf document that when it opens, runs through a loop loading specified xml data files, and saves the files, with the saving done by a trusted function.  All that works perfectly. All I want to add is applying a security policy to make the files read only. My .js function looks like this:

var ETASaveAs = app.trustedFunction( function(oDoc,cPath,cFlName)

{

    var aMyPath = oDoc.path.split("/");

    // Remove old file name

    aMyPath.pop();

    // put subdir path

    aMyPath.push(cPath);

    // Add new file name

    aMyPath.push(cFlName);

    app.beginPriv();

        // apply security policy       

            var oMyPolicy = null;

            var aPols = security.getSecurityPolicies()

            for(var index=0;index<aPols.length;index++){

                //console.println("policy " + index + " is " + aPols[index].name    );

                if(aPols[index].name == "prevent_changes"){

                    oMyPolicy = aPols[index]; break;

                }

            }

            if(oMyPolicy == null){ app.alert("Policy Not Found"); return; }

            // Now, Apply the security Policy

            var rtn = oDoc.encryptUsingPolicy({oPolicy: oMyPolicy });

            if(rtn.errorCode != 0) app.alert("Encrypt Security Error: " + rtn.errorText);

           // Put path path back together and save document

        var rtn = oDoc.saveAs(aMyPath.join("/"),"","",true );  // save as copy, otherwise active doc will be in subdirectory

            if(rtn.errorCode != 0) app.alert("Error saving: " + rtn.errorText);

    app.endPriv();

});

The problem is that the line

            var rtn = oDoc.encryptUsingPolicy({oPolicy: oMyPolicy });

gives: "NotAllowedError: Security settings prevent access to this property or method."

From the docs I can't see that the restrictions for encryptUsingPolicy are any different from saveAs, which works just fine, so I'm not sure what the problem is.

I'm using Acrobat Professional 9.5.

Thanks,

Jon