diff --git a/src/Symfony/Component/Process/Process.php b/src/Symfony/Component/Process/Process.php
index 98fa66a940..32704df3f1 100644
--- a/src/Symfony/Component/Process/Process.php
+++ b/src/Symfony/Component/Process/Process.php
@@ -491,6 +491,10 @@ class Process
      */
     public function getIncrementalOutput()
     {
+        if ($this->outputDisabled) {
+            throw new LogicException('Output has been disabled.');
+        }
+
         $this->requireProcessIsStarted(__FUNCTION__);
 
         $latest = stream_get_contents($this->stdout, -1, $this->incrementalOutputOffset);
@@ -510,7 +514,8 @@ class Process
      */
     public function clearOutput()
     {
-        $this->stdout = '';
+        ftruncate($this->stdout, 0);
+        fseek($this->stdout, 0);
         $this->incrementalOutputOffset = 0;
 
         return $this;
@@ -555,6 +560,10 @@ class Process
      */
     public function getIncrementalErrorOutput()
     {
+        if ($this->outputDisabled) {
+            throw new LogicException('Output has been disabled.');
+        }
+
         $this->requireProcessIsStarted(__FUNCTION__);
 
         $latest = stream_get_contents($this->stderr, -1, $this->incrementalErrorOutputOffset);
@@ -574,7 +583,8 @@ class Process
      */
     public function clearErrorOutput()
     {
-        $this->stderr = '';
+        ftruncate($this->stderr, 0);
+        fseek($this->stderr, 0);
         $this->incrementalErrorOutputOffset = 0;
 
         return $this;