diff --git a/ace.d.ts b/ace.d.ts
index 98201ccd59e..a65fa936d28 100644
--- a/ace.d.ts
+++ b/ace.d.ts
@@ -756,7 +756,7 @@ export namespace Ace {
     setStyle(style: string): void;
     unsetStyle(style: string): void;
     getFontSize(): string;
-    setFontSize(size: string): void;
+    setFontSize(size: number): void;
     focus(): void;
     isFocused(): boolean;
     blur(): void;