AngularJS Interpolation Error

I am displaying the properties of a room as part of a room management application I am working on, this is the output:

As you can see, the value of Beamer (Projector in english) is "Sony (lamp 01/12/2013)". This output is correct but when I open my console I see some errors concerning interpolation:

Larger resolution:

"Can't interpolate: {{getBeamerString()}} TypeError: Cannot read property 'aanwezig' of undefined"

This is my html partial:

<section class="eigenschappen">
    <table class="table table-striped table-hover table-bordered">
        <thead>
        <tr>
            <th>Eigenschap</th>
            <th>Waarde</th>
            <th>Bewerk</th>
        </tr>
        </thead>
        <tbody>

        ...

        <tr>
            <td><img class="ruimteNaam" src="../Images/Beamer.png" alt=""/>Beamer</td>
            <td>{{getBeamerString()}}</td>
            <td><img class="edit" src="../Images/Edit.png" alt=""/></td>
        </tr>

        ...

        </tbody>
    </table>
</section>

currentRuimte (CurrentRoom in English) is a scope defined variable that gets its value using a resource service that gets that data from my mongoDB:

function RuimteController($scope, $routeParams, Ruimte) {

    $scope.currentRuimte = Ruimte.get({
        ruimteNaam: $routeParams.ruimteNaam
    });

    $scope.getBeamerString = function () {
        var beamer = $scope.currentRuimte.beamer;
        if(beamer.aanwezig == 'Ja'){
            return beamer.typeBeamer + ' (lamp: ' + beamer.datumLamp + ')';
        }else{
            return '-';
        }
    }
}

When I inspect the scope using batarang I get this:

Why do I get the error and why do I still get the correct output? Is there a way to prevent the error from happening? Please correct me as much as you can, I am fairly new to AngularJS and Javascript in general and trying to expand my skillset.