diff --git a/.travis.yml b/.travis.yml
index 2be2fb1..2f4e3a1 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,8 +2,9 @@ sudo: required
 language: generic
 services: docker
 env:
-- HHVM_VERSION=4.41-latest
-- HHVM_VERSION=4.51-latest
+- HHVM_VERSION=4.52-latest
+- HHVM_VERSION=latest
+- HHVM_VERSION=nightly
 install:
 - docker pull hhvm/hhvm:$HHVM_VERSION
 script:
diff --git a/composer.json b/composer.json
index 4cf0a8f..c4a1b39 100644
--- a/composer.json
+++ b/composer.json
@@ -10,7 +10,7 @@
     "hhvm": "^4.41",
     "hhvm/hsl": "^4.0",
     "hhvm/type-assert": "^3.2",
-    "hhvm/hsl-experimental": "^4.50"
+    "hhvm/hsl-experimental": "^4.52"
   },
   "require-dev": {
     "hhvm/hhast": "^4.0.2",
diff --git a/src/TestLib/StringOutput.hack b/src/TestLib/StringOutput.hack
index ac4519d..d683929 100644
--- a/src/TestLib/StringOutput.hack
+++ b/src/TestLib/StringOutput.hack
@@ -28,8 +28,8 @@ final class StringOutput implements IO\WriteHandle, IO\UserspaceHandle {
   public async function writeAsync(
     string $data,
     ?num $_timeout_mixed = null,
-  ): Awaitable<void> {
-    $this->write($data);
+  ): Awaitable<int> {
+    return $this->write($data);
   }
 
   public function isEndOfFile(): bool {