AngularJs filter "Error while interpolating"

AngularJs filter "Error while interpolating"

Inside my AngularJs filter I have the following:
return function(list){
return list;
}
That works fine
However, the following throws an error of "Error while interpolating {{
list | filterName}}...TypeError: list is undefined"
return function(list){
return list.length;
}
What is happening that is causing list to be undefined for a brief moment
when this function runs, or what can I do to fix this issue. The value
still gets returned, I just have a nasty error in console.