diff --git a/ext/fg/js/source.js b/ext/fg/js/source.js
index bbf00e30..a360b331 100644
--- a/ext/fg/js/source.js
+++ b/ext/fg/js/source.js
@@ -180,7 +180,7 @@ class TextSourceRange {
             let consumed = 0;
             let stripped = 0;
             while (state.remainder - consumed > 0) {
-                const currentChar = node.nodeValue[offset - consumed - stripped]; // negative indices are undefined in JS
+                const currentChar = node.nodeValue[offset - 1 - consumed - stripped]; // negative indices are undefined in JS
                 if (!currentChar) {
                     break;
                 } else if (currentChar.match(IGNORE_TEXT_PATTERN)) {