Angular filter works, but sees input as undefined and throws error

I wrote a filter that appears to be working fine, although I am getting the following error.

TypeError: Cannot read property 'replaceAll' of undefined

The filter replaces erroneous relative urls with absolute ones. Here is the filter and markup.

//located elsewhere
String.prototype.replaceAll = function(s,r){
    return this.split(s).join(r);
}
//located elsewhere
var imgBase = '<img src="http://discourse.mysite.com';

app.filter('prependImg_topic', function(){
  return function(cooked) {
    return cooked.replaceAll('<img src="//discourse.mysite.com', imgBase);
  }
})

Markup:

<div class="topic-post-body" ng-bind-html="topic.post_stream.posts[0].cooked | prependImg_topic "></div> "

I have an inclination that I need to use $sce because I am inputting the data in ng-bind-html. If I print the value like so: {{topic.post_stream.posts[0].cooked}} , it exists, so I know it is not undefined. In addition, the filter works and properly writes urls. Go figure.

I can provide any additional details needed to help answer. Thanks.

EDIT: Another oddity that may help is that inside of a ng-repeat this filter works, and throws no errors. Trippy stuff!

<div class="post" ng-repeat="post in posts" ng-if="$index > 0">
     <div class="topic-post-body" ng-bind-html="post.cooked | prependImg_topic"></div>
</div>