diff --git a/api.include.php b/api.include.php
index 730c32dd..f3676846 100644
--- a/api.include.php
+++ b/api.include.php
@@ -2268,7 +2268,7 @@ public function withoutAttribute($attribute): ServerRequestInterface
      */
     class Stream implements StreamInterface
     {
-        function __toString() {}
+        function __toString():string { return ""; }
 
         /** @var resource|null A resource reference */
         private $stream;
diff --git a/api.php b/api.php
index 4422502f..47cb1386 100644
--- a/api.php
+++ b/api.php
@@ -2268,7 +2268,7 @@ public function withoutAttribute($attribute): ServerRequestInterface
      */
     class Stream implements StreamInterface
     {
-        function __toString() {}
+        function __toString():string { return ""; }
 
         /** @var resource|null A resource reference */
         private $stream;
diff --git a/build.php b/build.php
index 3dfd9007..8122f161 100644
--- a/build.php
+++ b/build.php
@@ -109,7 +109,7 @@ function run(string $base, array $dirs, string $filename, array $ignore, array $
 ];
 
 $replaces = [
-    'use StreamTrait;' => 'function __toString() {}'
+    'use StreamTrait;' => 'function __toString():string { return ""; }'
 ];
 
 $directories = ['vendor/nyholm', 'src'];