c# - API returning 500 error after changing one property from guid to string -

i have api returns list of objects. object has field of type ienumerable<string>. field populated list of guid strings, set in sitecore multilist:

characteristics.vertwilt = item.fields.get("vert wilt") != null ? item.fields.get("vert wilt").split(',').select(f =>f) : null; 

however, want instead retrieve values of multilist items, not guid ids, because don't want guids displaying on page. changed line this:

characteristics.vertwilt = item.fields.get("vert wilt") != null ? item.fields.get("vert wilt").split(',').select(f => service.getitem<idropdownitem>(guid.parse(f)).value) : null; 

my code executes without error. api code:

[httppost] [httproute("applyfilters")] public httpresponsemessage applyfilters([frombody] filtermodel filters) {     varietyselectordatahelper helper = new varietyselectordatahelper();      var data = helper.searchvarieties(filters).select(i => new varietymodel(i)).orderby(v => v.sortorder);     return request.createresponse(httpstatuscode.ok, data); } 

data populated should be, , field characteristics.vertwilt looks option1, option2, option3 instead of 3f9f1afc-4dd7-4445-83e2-ccbbb62bff78,8a40af0e-3f79-4060-bd9d-ee0a29cb1d52,b596a791-5917-45b6-bb2f-fe433fee6039

however, in console see 500 error, , data not display on page. used firebug inspect error.

"message": "an error has occurred.", "exceptionmessage": "guid should contain 32 digits 4 dashes (xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx )." 

why expecting guid? need changed?