diff --git a/lib/utils/conversion-utils.js b/lib/utils/conversion-utils.js
--- a/lib/utils/conversion-utils.js
+++ b/lib/utils/conversion-utils.js
@@ -16,7 +16,7 @@
   outputValidator: TType<T>,
   data: T,
 ): T {
-  const conversionFunction = id => {
+  const conversionFunction = (id: string) => {
     if (id.indexOf('|') !== -1) {
       console.warn(`Server id '${id}' already has a prefix`);
       return id;
@@ -33,7 +33,7 @@
   data: T,
 ): T {
   const prefix = serverPrefixID + '|';
-  const conversionFunction = id => {
+  const conversionFunction = (id: string) => {
     if (id.startsWith(prefix)) {
       return id.substr(prefix.length);
     }