diff --git a/src/components/DocumentView/CodeBlock/highlight.ts b/src/components/DocumentView/CodeBlock/highlight.ts
index 66876594dc..63eda76bcc 100644
--- a/src/components/DocumentView/CodeBlock/highlight.ts
+++ b/src/components/DocumentView/CodeBlock/highlight.ts
@@ -312,7 +312,7 @@ function cleanupLine(line: string): string {
     return line.replace(/\r/g, '');
 }
 
-let memoryHighlighter = undefined;
+let memoryHighlighter: HighlighterGeneric<any, any> | undefined = undefined;
 
 const loadHighlighterSingleton = singleton(async () => {
     return await trace('highlighting.loadHighlighter', async () => {