diff --git a/static/brush-editor.js b/static/brush-editor.js
index 38d43df..e88e3af 100644
--- a/static/brush-editor.js
+++ b/static/brush-editor.js
@@ -102,7 +102,7 @@ export class BrushEditor extends HTMLElement {
 
         for (let diagnostic of diagnostics) {
             let firstLine = lineMap.lineIndexAt(diagnostic.start);
-            let lastLine = lineMap.lineIndexAt(diagnostic.start);
+            let lastLine = lineMap.lineIndexAt(diagnostic.end);
             for (let i = firstLine; i <= lastLine; ++i) {
                 let bounds = lineMap.get(i);
                 let start = i == firstLine ? diagnostic.start - bounds.start : 0;