Handlebars variable is undefined when used inside custom helper - handlebars.js

Question

I am using Handlebars in combination with express.js and registered a custom helper:
let hbs = exphbs.create({
extname: 'hbs',
defaultLayout: 'layout',
layoutsDir: path.join(__dirname, 'views/'),
partialsDir: [
path.join(__dirname, 'views/modals/'),
path.join(__dirname, 'views/partials/'),
path.join(__dirname, 'views/charts/'),
path.join(__dirname, 'views/customScripts/'),
],
helpers: {
when: function(operand_1, operator, operand_2, options){
// Use with:
// {{#when <operand1> 'eq' <operand2>}}
console.log("Using the when helper:");
console.log("operand_1: " + operand_1 + " operator: " + operator + " operand_2: " + operand_2);
let operators = {
'eq': function (l, r) {
return l == r;
},
'noteq': function (l, r) {
return l != r;
},
'gt': function (l, r) {
return Number(l) > Number(r);
},
'or': function (l, r) {
return l || r;
},
'and': function (l, r) {
return l && r;
},
'%': function (l, r) {
return (l % r) === 0;
}
}
, result = operators[operator](operand_1, operand_2);
if(result) return options.fn(this);
else return options.inverse(this);
}
}
In my file I have the following HTML code:
<div class="col-lg-2">
<label for="assetSelection">
Asset:
</label>
{{currentFormSelectionAsset}} <-- This works
<select class="form-control" id="assetSelection" name="assetSelection">
{{#each assetList}}
<option value="{{this.id}}" {{#when currentFormSelectionAsset 'eq' this.id}}selected{{/when}}>{{this.name}}</option>
{{/each}}
</select>
</div>
Calling my html file I only get the following output in my console:
Using the when:
operand_1: undefined operator: eq operand_2: 2
It looks like I cannot pass the variable into my custom helper but due to the fact that I can print it (one line above the select) makes sure that the variable is present and works.
Is there a limitation about using variables inside custom helper or am I missing something?