diff --git a/src/Internal/Posix/PosixRunner.php b/src/Internal/Posix/PosixRunner.php
index 7e244ef..91d7e94 100644
--- a/src/Internal/Posix/PosixRunner.php
+++ b/src/Internal/Posix/PosixRunner.php
@@ -52,8 +52,9 @@ public function start(
             $command
         );
 
-        // Errors checked below, so suppressing all errors during call to proc_open() and $this->generateFds().
-        \set_error_handler(static fn () => true);
+        \set_error_handler(static function (int $code, string $message): never {
+            throw new ProcessException("Process could not be started: Errno: {$code}; {$message}");
+        });
 
         try {
             $proc = \proc_open(
@@ -69,14 +70,16 @@ public function start(
         }
 
         if (!\is_resource($proc)) {
-            $message = "Could not start process";
-            if ($error = \error_get_last()) {
-                $message .= \sprintf(" Errno: %d; %s", $error["type"], $error["message"]);
-            }
-            throw new ProcessException($message);
+            throw new ProcessException("Process could not be started: unknown error");
         }
 
         $extraDataPipe = $pipes[3];
+
+        /** @psalm-suppress TypeDoesNotContainType */
+        if (!\is_resource($extraDataPipe)) {
+            throw new ProcessException("Process could not be started: the data pipe closed unexpectedly");
+        }
+
         \stream_set_blocking($extraDataPipe, false);
 
         $suspension = EventLoop::getSuspension();
@@ -106,9 +109,11 @@ static function (CancelledException $e) use ($suspension, $callbackId): void {
             $cancellation->unsubscribe($cancellationId);
         }
 
-        $pid = \rtrim(\fgets($extraDataPipe));
+        $pid = \rtrim(\fgets($extraDataPipe) ?: '');
         if (!$pid || !\is_numeric($pid)) {
-            throw new ProcessException("Could not determine PID");
+            \proc_terminate($proc);
+            \proc_close($proc);
+            throw new ProcessException("Process could not be started: could not determine PID");
         }
 
         $stdin = new WritableResourceStream($pipes[0]);
diff --git a/src/Internal/Windows/WindowsRunner.php b/src/Internal/Windows/WindowsRunner.php
index 6dcd4ec..9a3fe11 100644
--- a/src/Internal/Windows/WindowsRunner.php
+++ b/src/Internal/Windows/WindowsRunner.php
@@ -56,21 +56,25 @@ public function start(
 
         $options['bypass_shell'] = true;
 
-        $proc = @\proc_open(
-            $this->makeCommand($workingDirectory ?? ''),
-            self::FD_SPEC,
-            $pipes,
-            $workingDirectory ?: null,
-            $environment ?: null,
-            $options
-        );
+        \set_error_handler(static function (int $code, string $message): never {
+            throw new ProcessException("Process could not be started: Errno: {$code}; {$message}");
+        });
+
+        try {
+            $proc = \proc_open(
+                $this->makeCommand($workingDirectory ?? ''),
+                self::FD_SPEC,
+                $pipes,
+                $workingDirectory ?: null,
+                $environment ?: null,
+                $options
+            );
+        } finally {
+            \restore_error_handler();
+        }
 
         if (!\is_resource($proc)) {
-            $message = "Could not start process";
-            if ($error = \error_get_last()) {
-                $message .= \sprintf(" Errno: %d; %s", $error["type"], $error["message"]);
-            }
-            throw new ProcessException($message);
+            throw new ProcessException("Process could not be started: unknown error");
         }
 
         $status = \proc_get_status($proc);
@@ -84,6 +88,7 @@ public function start(
 
         if ($written !== SocketConnector::SECURITY_TOKEN_SIZE * 6 + \strlen($command) + 2) {
             \fclose($pipes[2]);
+            \proc_terminate($proc);
             \proc_close($proc);
 
             throw new ProcessException("Could not send security tokens / command to process wrapper");