diff --git a/src/function/predefined.js b/src/function/predefined.js
index f868ba4..1c38c26 100644
--- a/src/function/predefined.js
+++ b/src/function/predefined.js
@@ -3,7 +3,7 @@ const h2p = require('html2plaintext');
 
 const namespace = 'http://sti2.at#';
 
-const toUpperCase = (data) => data[0].toString().toUpperCase();
+const toUpperCase = ([str]) => str.toString().toUpperCase();
 
 const utmToLat = (data) => {
   const x = data[0];
diff --git a/tests/test.js b/tests/test.js
index 4e8589c..43e34ad 100644
--- a/tests/test.js
+++ b/tests/test.js
@@ -324,7 +324,7 @@ it('Predefined option parameter function mapping', async () => {
   const options = {
     functions: {
       'http://users.ugent.be/~bjdmeest/function/grel.ttl#toLowerCase': function (data) {
-        return data[0].toString().toLowerCase();
+        return data.toString().toLowerCase();
       },
     },
   };
@@ -846,8 +846,8 @@ it('subjFuncMap 2', async () => {
   let i = 0;
   const options = {
     functions: {
-      'http://myfunc.com/getId': function (data) {
-        return `http://example.com/${i++}/${data[0]}`;
+      'http://myfunc.com/getId': function ([str]) {
+        return `http://example.com/${i++}/${str}`;
       },
     },
   };