diff --git a/material-ui/material-ui.d.ts b/material-ui/material-ui.d.ts
index 1a060aed81..38b6c4be22 100644
--- a/material-ui/material-ui.d.ts
+++ b/material-ui/material-ui.d.ts
@@ -1491,7 +1491,7 @@ declare namespace __MaterialUI {
         style?: React.CSSProperties;
     }
     export class LeftNav extends React.Component<LeftNavProps, {}> {
-      toggle();
+      toggle(): void;
     }
 
     interface LinearProgressProps extends React.Props<LinearProgress> {