diff --git a/src/functions.php b/src/functions.php
index 6a4c9a1..b74494f 100644
--- a/src/functions.php
+++ b/src/functions.php
@@ -67,8 +67,12 @@ function buffer(ReadableStreamInterface $stream, $maxLength = null)
 
         $stream->on('data', $bufferer);
 
-        $stream->on('error', function ($error) use ($reject) {
-            $reject(new \RuntimeException('An error occured on the underlying stream while buffering', 0, $error));
+        $stream->on('error', function (\Exception $e) use ($reject) {
+            $reject(new \RuntimeException(
+                'An error occured on the underlying stream while buffering: ' . $e->getMessage(),
+                $e->getCode(),
+                $e
+            ));
         });
 
         $stream->on('close', function () use ($resolve, &$buffer) {
@@ -78,7 +82,7 @@ function buffer(ReadableStreamInterface $stream, $maxLength = null)
         $reject(new \RuntimeException('Cancelled buffering'));
     });
 
-    return $promise->then(null, function ($error) use (&$buffer, $bufferer, $stream) {
+    return $promise->then(null, function (\Exception $error) use (&$buffer, $bufferer, $stream) {
         // promise rejected => clear buffer and buffering
         $buffer = '';
         $stream->removeListener('data', $bufferer);
@@ -140,9 +144,13 @@ function first(EventEmitterInterface $stream, $event = 'data')
         $stream->on($event, $listener);
 
         if ($event !== 'error') {
-            $stream->on('error', function ($error) use ($stream, $event, $listener, $reject) {
+            $stream->on('error', function (\Exception $e) use ($stream, $event, $listener, $reject) {
                 $stream->removeListener($event, $listener);
-                $reject(new \RuntimeException('An error occured on the underlying stream while waiting for event', 0, $error));
+                $reject(new \RuntimeException(
+                    'An error occured on the underlying stream while waiting for event: ' . $e->getMessage(),
+                    $e->getCode(),
+                    $e
+                ));
             });
         }
 
@@ -207,8 +215,12 @@ function all(EventEmitterInterface $stream, $event = 'data')
     $stream->on($event, $bufferer);
 
     $promise = new Promise\Promise(function ($resolve, $reject) use ($stream, &$buffer) {
-        $stream->on('error', function ($error) use ($reject) {
-            $reject(new \RuntimeException('An error occured on the underlying stream while buffering', 0, $error));
+        $stream->on('error', function (\Exception $e) use ($reject) {
+            $reject(new \RuntimeException(
+                'An error occured on the underlying stream while buffering: ' . $e->getMessage(),
+                $e->getCode(),
+                $e
+            ));
         });
 
         $stream->on('close', function () use ($resolve, &$buffer) {
diff --git a/tests/AllTest.php b/tests/AllTest.php
index b935c93..069bd39 100644
--- a/tests/AllTest.php
+++ b/tests/AllTest.php
@@ -86,9 +86,13 @@ public function testEmittingErrorOnStreamRejects()
         $stream = new ThroughStream();
         $promise = Stream\all($stream);
 
-        $stream->emit('error', array(new \RuntimeException('test')));
+        $stream->emit('error', array(new \RuntimeException('test', 42)));
 
-        $this->expectPromiseReject($promise);
+        $promise->then(null, $this->expectCallableOnceWith(new \RuntimeException(
+            'An error occured on the underlying stream while buffering: test',
+            42,
+            new \RuntimeException('test', 42)
+        )));
     }
 
     public function testEmittingErrorAfterEmittingDataOnStreamRejects()
@@ -97,9 +101,13 @@ public function testEmittingErrorAfterEmittingDataOnStreamRejects()
         $promise = Stream\all($stream);
 
         $stream->emit('data', array('hello', $stream));
-        $stream->emit('error', array(new \RuntimeException('test')));
+        $stream->emit('error', array(new \RuntimeException('test', 42)));
 
-        $this->expectPromiseReject($promise);
+        $promise->then(null, $this->expectCallableOnceWith(new \RuntimeException(
+            'An error occured on the underlying stream while buffering: test',
+            42,
+            new \RuntimeException('test', 42)
+        )));
     }
 
     public function testCancelPendingStreamWillReject()
diff --git a/tests/BufferTest.php b/tests/BufferTest.php
index 567d26a..45bc06c 100644
--- a/tests/BufferTest.php
+++ b/tests/BufferTest.php
@@ -55,9 +55,13 @@ public function testEmittingErrorOnStreamRejects()
         $stream = new ThroughStream();
         $promise = Stream\buffer($stream);
 
-        $stream->emit('error', array(new \RuntimeException('test')));
+        $stream->emit('error', array(new \RuntimeException('test', 42)));
 
-        $this->expectPromiseReject($promise);
+        $promise->then(null, $this->expectCallableOnceWith(new \RuntimeException(
+            'An error occured on the underlying stream while buffering: test',
+            42,
+            new \RuntimeException('test', 42)
+        )));
     }
 
     public function testEmittingErrorAfterEmittingDataOnStreamRejects()
@@ -66,9 +70,13 @@ public function testEmittingErrorAfterEmittingDataOnStreamRejects()
         $promise = Stream\buffer($stream);
 
         $stream->emit('data', array('hello', $stream));
-        $stream->emit('error', array(new \RuntimeException('test')));
+        $stream->emit('error', array(new \RuntimeException('test', 42)));
 
-        $this->expectPromiseReject($promise);
+        $promise->then(null, $this->expectCallableOnceWith(new \RuntimeException(
+            'An error occured on the underlying stream while buffering: test',
+            42,
+            new \RuntimeException('test', 42)
+        )));
     }
 
     public function testCancelPendingStreamWillReject()
diff --git a/tests/FirstTest.php b/tests/FirstTest.php
index 04eb7c8..1ce643c 100644
--- a/tests/FirstTest.php
+++ b/tests/FirstTest.php
@@ -83,9 +83,13 @@ public function testEmittingErrorOnStreamWillReject()
         $stream = new ThroughStream();
         $promise = Stream\first($stream);
 
-        $stream->emit('error', array(new \RuntimeException('test')));
+        $stream->emit('error', array(new \RuntimeException('test', 42)));
 
-        $this->expectPromiseReject($promise);
+        $promise->then(null, $this->expectCallableOnceWith(new \RuntimeException(
+            'An error occured on the underlying stream while waiting for event: test',
+            42,
+            new \RuntimeException('test', 42)
+        )));
     }
 
     public function testEmittingErrorResolvesWhenWaitingForErrorEvent()
diff --git a/tests/TestCase.php b/tests/TestCase.php
index a2a8490..2114f1c 100644
--- a/tests/TestCase.php
+++ b/tests/TestCase.php
@@ -22,18 +22,7 @@ protected function expectCallableOnceWith($value)
         $mock
             ->expects($this->once())
             ->method('__invoke')
-            ->with($this->equalTo($value));
-
-        return $mock;
-    }
-
-    protected function expectCallableOnceParameter($type)
-    {
-        $mock = $this->createCallableMock();
-        $mock
-            ->expects($this->once())
-            ->method('__invoke')
-            ->with($this->isInstanceOf($type));
+            ->with($value);
 
         return $mock;
     }