diff --git a/types/gulp-istanbul/gulp-istanbul-tests.ts b/types/gulp-istanbul/gulp-istanbul-tests.ts
index a57932419d..af15f5df54 100644
--- a/types/gulp-istanbul/gulp-istanbul-tests.ts
+++ b/types/gulp-istanbul/gulp-istanbul-tests.ts
@@ -5,7 +5,7 @@ function testFramework(): NodeJS.ReadWriteStream {
     return null;
 }
 
-gulp.task('test', function (cb: Function) {
+gulp.task('test', function (cb: () => void) {
     gulp.src(['lib/**/*.js', 'main.js'])
         .pipe(istanbul()) // Covering files
         .pipe(gulp.dest('test-tmp/'))
@@ -17,7 +17,7 @@ gulp.task('test', function (cb: Function) {
         });
 });
 
-gulp.task('test', function (cb: Function) {
+gulp.task('test', function (cb: () => void) {
     gulp.src(['lib/**/*.js', 'main.js'])
         .pipe(istanbul({includeUntested: true})) // Covering files
         .pipe(istanbul.hookRequire())
@@ -29,7 +29,7 @@ gulp.task('test', function (cb: Function) {
         });
 });
 
-gulp.task('test', function (cb: Function) {
+gulp.task('test', function (cb: () => void) {
     gulp.src(['lib/**/*.js', 'main.js'])
         .pipe(istanbul({includeUntested: true})) // Covering files
         .pipe(istanbul.hookRequire())
diff --git a/types/gulp-watch/gulp-watch-tests.ts b/types/gulp-watch/gulp-watch-tests.ts
index 0260a24bb6..de531257a5 100644
--- a/types/gulp-watch/gulp-watch-tests.ts
+++ b/types/gulp-watch/gulp-watch-tests.ts
@@ -7,7 +7,7 @@ gulp.task('stream', () =>
         .pipe(gulp.dest('build'))
 );
 
-gulp.task('callback', (cb: Function) =>
+gulp.task('callback', (cb: () => void) =>
     watch('css/**/*.css', () =>
         gulp.src('css/**/*.css')
             .pipe(watch('css/**/*.css'))