Cryptic message when serializing Computed Observable to JSON

If you have an error in one of your computed observable in Knockout.js, the toJson function will set the value of that property to a cryptic script snipped when serializing it to JSON. For example:

var SortField = function (expression) {
        this.FieldDef = ko.observable(null);
        this.SortOrder = ko.observable("");

        this.ExpressionType = "SortField";

        this.Label = ko.computed(function () {
            return this.FieldDef() == null? "" : this.FieldDef().Label + ' ' + this.SortOrder();
        }, this);

    }
var FieldDefinition = function (table_name, field_name, table_alias) {
        this.TableName = table_name;
        this.TableAlias = ko.observable(table_alias);
        this.FieldName = field_name;
        this.Label = ko.computed(function () {
            return this.TableAlias() + '⇨' + field_name;
        }, this);

In this example SortField’s Label function is not calling the Label function properly from the FieldDef property. If you serialize to JSON, you will get  the following:

function dependentObservable() {\n        if (arguments.length > 0) {\n
 if (typeof writeFunction === \"function\") {\n
     // Writing a value\n
     writeFunction.apply(evaluatorFunctionTarget, arguments);\n
 } else {\n
     throw new Error(\"Cannot write a value to a ko.computed unless you specify a 'write' option. If you wish to read the current value, don't pass any parameters.\");\n
 }\n
 return this; // Permits chained assignments\n        } else {\n
 // Reading the value\n
 if (!_hasBeenEvaluated)\n
     evaluateImmediate();\n
 ko.dependencyDetection.registerDependency(dependentObservable);\n
 return _latestValue;\n        }\n    } ASC"}],"ExpressionType":"Hierarchy","Label":"function dependentObservable() {\n        if (arguments.length > 0) {\n
 if (typeof writeFunction === \"function\") {\n
     // Writing a value\n
     writeFunction.apply(evaluatorFunctionTarget, arguments);\n
 } else {\n
     throw new Error(\"Cannot write a value to a ko.computed unless you specify a 'write' option. If you wish to read the current value, don't pass any parameters.\");\n
 }\n
 return this; // Permits chained assignments\n        } else {\n
 // Reading the value\n
 if (!_hasBeenEvaluated)\n
     evaluateImmediate();\n
 ko.dependencyDetection.registerDependency(dependentObservable);\n
 return _latestValue;\n        }\n    }

For the value of the Label property of the SortField.

The fix is to call the Label function as this.FieldDef().Label() inside SortField's Label.