diff --git a/question/type/multianswer/renderer.php b/question/type/multianswer/renderer.php
index a959080ebf0..d67d0fc8620 100644
--- a/question/type/multianswer/renderer.php
+++ b/question/type/multianswer/renderer.php
@@ -189,9 +189,9 @@ class qtype_multianswer_textfield_renderer extends qtype_multianswer_subq_render
         }
 
         // Work out a good input field size.
-        $size = max(1, strlen(trim($response)) + 1);
+        $size = max(1, textlib::strlen(trim($response)) + 1);
         foreach ($subq->answers as $ans) {
-            $size = max($size, strlen(trim($ans->answer)));
+            $size = max($size, textlib::strlen(trim($ans->answer)));
         }
         $size = min(60, round($size + rand(0, $size*0.15)));
         // The rand bit is to make guessing harder.