diff --git a/src/shadow.tsx b/src/shadow.tsx
index 6c82d55..02c68e6 100644
--- a/src/shadow.tsx
+++ b/src/shadow.tsx
@@ -24,7 +24,7 @@ function buildShadow(highlights: Highlight[], text: string): ReactNode[] {
     } else if (state == ShadowState.Highlight) {
       if (remaining_highlights.length > 0 && i == remaining_highlights[0].end) {
         // End the span
-        output.push(<span className="highlighted">{buffer}</span>);
+        output.push(<span key={i} className="highlighted">{buffer}</span>);
         buffer = chr;
         state = ShadowState.Text;
         remaining_highlights = remaining_highlights.slice(1);