Async issue?

CatalogAPIManager.swift

func sendEmail(email: Email, completion: SendEmailCompletionBlock) -> Request {
        let fields: JSONDict = [
            "first_name"        : email.firstName,
            "last_name"         : email.lastName,
            "email_address"     : email.emailAddress,
            "ip_address"        : email.ipAddress,
            "vendor_id"         : email.vendorID,
            "event_date_epoch"  : email.eventDateEpoch,
            "message"           : email.message,
            "phone"             : email.phoneNumber
        ]
        
        let jsonData = NSJSONSerialization.dataWithJSONObject(fields, options: .allZeros, error: nil)!
        let payload = NSString(data: jsonData, encoding: NSASCIIStringEncoding)!
        let parameters: JSONDict = [
            "partner_key" : kApiKey,
            "payload" : payload
        ]
            
        // This works
        return APIManager.performRequest(queue: nil, method: .GET, version: .V3, path: "OpenCreateVendorLead", parameters: parameters, completion: { (data, error) -> Void in
            dispatch_async(dispatch_get_main_queue()) {
                if error != nil {
                    completion(success: false, error: error)
                } else {
                    completion(success: true, error: nil)
                }
            }
        })
        
        // This doesn't:
        // Command /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swiftc failed with exit code 1
        return APIManager.performRequest(queue: nil, method: .GET, version: .V3, path: "OpenCreateVendorLead", parameters: parameters, completion: { (data, error) -> Void in
            Async.main {
                if error != nil {
                    completion(success: false, error: error)
                } else {
                    completion(success: true, error: nil)
                }
            }
        })
        
        // And this doesn't either
        return APIManager.performRequest(queue: nil, method: .GET, version: .V3, path: "OpenCreateVendorLead", parameters: parameters, completion: { (data, error) -> Void in
            Async.main({ () -> Void in
                if error != nil {
                    completion(success: false, error: error)
                } else {
                    completion(success: true, error: nil)
                }
            })
        })

    }

After investigation, this was determined to be an issue with implicit returns being contained in a block that expects to return void.