Updating an Existing Property on Workflow vote


After a user votes on a workflow, I want to update an existing property (_conversation_history). The code below is what I am using.

It appears that the code is generating the correct string in the variable (newconvo)), but it is not updating the property on the item?

Innovator inn = this.getInnovator();
Item controlledItem = this.apply("GetControlledItem");
if (controlledItem.isError()) {
return inn.newError("smc_TechInquiry_Notifications: Retrive Workflow: Unexpected error: " + controlledItem.getErrorString());
// get the message from requestor and reply to requestor

string messagefrom = controlledItem.getProperty("_message_from_requestor", "");
string replyTo = controlledItem.getProperty("_reply_to_requestor", "");
string history = controlledItem.getProperty("_conversation_history","");

string newconvo = history + " " + messagefrom + " " + replyTo;

string message = controlledItem.getPropertyCondition("_conversation_history");

return this;

Thank you for your help!