diff --git a/frontend/static/challenges/sourcecode.txt b/frontend/static/challenges/sourcecode.txt
index 5f68d45e1..02e1bff1d 100644
--- a/frontend/static/challenges/sourcecode.txt
+++ b/frontend/static/challenges/sourcecode.txt
@@ -22235,7 +22235,7 @@ class WordGenerator extends Wordset {
           this.ngrams[prefix] = new CharDistribution();
         }
         this.ngrams[prefix].addChar(c);
-        prefix = (prefix + c).slice(-prefixSize);
+        prefix = (prefix + c).substr(-prefixSize);
       }
     }
   }
@@ -22243,7 +22243,7 @@ class WordGenerator extends Wordset {
   randomWord() {
     let word = "";
     for (;;) {
-      const prefix = word.slice(-prefixSize);
+      const prefix = word.substr(-prefixSize);
       let charDistribution = this.ngrams[prefix];
       if (!charDistribution) {
         // This shouldn't happen if this.ngrams is complete. If it does