diff --git a/roslibjs/roslibjs.d.ts b/roslibjs/roslibjs.d.ts
index 4614deb92..667a37983 100644
--- a/roslibjs/roslibjs.d.ts
+++ b/roslibjs/roslibjs.d.ts
@@ -9,8 +9,8 @@ declare module ROSLIB {
 			url: string
 		});
 
-		on(eventName: string, callback: (event: any) => void);
-		connect(url: string);
+		on(eventName: string, callback: (event: any) => void) : void;
+		connect(url: string) : void;
 	}
 
 
@@ -21,4 +21,4 @@ declare module ROSLIB {
 			serviceType: string
 		});
 	}
-}
\ No newline at end of file
+}