diff -up ./src/WritableResourceStream.php.php8 ./src/WritableResourceStream.php
--- ./src/WritableResourceStream.php.php8	2021-04-07 16:22:09.048553209 +0200
+++ ./src/WritableResourceStream.php	2021-04-07 16:22:14.797526978 +0200
@@ -117,10 +117,15 @@ final class WritableResourceStream exten
             $error = $errstr;
         });
 
-        if ($this->writeChunkSize === -1) {
-            $sent = \fwrite($this->stream, $this->data);
+        if (is_resource($this->stream)) {
+           if ($this->writeChunkSize === -1) {
+               $sent = \fwrite($this->stream, $this->data);
+           } else {
+               $sent = \fwrite($this->stream, $this->data, $this->writeChunkSize);
+           }
         } else {
-            $sent = \fwrite($this->stream, $this->data, $this->writeChunkSize);
+           $sent = 0;
+           $error = 'this is not a valid stream resource';
         }
 
         \restore_error_handler();
diff -up ./tests/WritableStreamResourceTest.php.php8 ./tests/WritableStreamResourceTest.php
--- ./tests/WritableStreamResourceTest.php.php8	2021-04-07 16:18:25.234574523 +0200
+++ ./tests/WritableStreamResourceTest.php	2021-04-07 16:18:44.985484393 +0200
@@ -508,7 +508,7 @@ class WritableResourceStreamTest extends
         $buffer->handleWrite();
 
         $this->assertInstanceOf('Exception', $error);
-        $this->assertSame('Unable to write to stream: fwrite(): send of 3 bytes failed with errno=32 Broken pipe', $error->getMessage());
+        $this->assertEqualsIgnoringCase('Unable to write to stream: fwrite(): send of 3 bytes failed with errno=32 Broken pipe', $error->getMessage());
     }
 
     private function createWriteableLoopMock()