No Insertion Point or Active Document

My find/change script steps through each document in a book as it executes.  At the start of the script, I read each doc filename in the book into an array and then use the SimpleOpen method to make each document active; something I picked from a post a few months ago.  However, I sometimes get an error that says, "No insertion point or active document.  Cannot continue."  Any thoughts on what I might do to fix this?  I searched the forum, but no luck.  Here's the code I'm using to open/activate each doc.

// Open each doc using doc.Name and call executeHitlist function
function searchDocs () {
    for (k = 1; k <= docCount; k++) {
        doc = SimpleOpen(openDocs[k]);
        $.writeln("Searching " + doc.Name + "...");
        doc = app.ActiveDoc;  
        executeHitlist ();
    }
}