diff --git a/reframe/utility/os_ext.py b/reframe/utility/os_ext.py
index 92d12bad3e..5c9e761169 100644
--- a/reframe/utility/os_ext.py
+++ b/reframe/utility/os_ext.py
@@ -25,8 +25,8 @@ def run_command(cmd, check=False, timeout=None, shell=False):
     except subprocess.TimeoutExpired as e:
         os.killpg(proc.pid, signal.SIGKILL)
         raise SpawnedProcessTimeout(e.cmd,
-                                    proc.stdout,
-                                    proc.stderr, timeout) from None
+                                    proc.stdout.read(),
+                                    proc.stderr.read(), timeout) from None
 
     completed = subprocess.CompletedProcess(args=shlex.split(cmd),
                                             returncode=proc.returncode,
diff --git a/unittests/test_utility.py b/unittests/test_utility.py
index 4cc794439c..3e63942a91 100644
--- a/unittests/test_utility.py
+++ b/unittests/test_utility.py
@@ -26,9 +26,12 @@ def test_command_error(self):
     def test_command_timeout(self):
         try:
             os_ext.run_command('sleep 3', timeout=2)
-            self.fail('Expected timeout')
         except SpawnedProcessTimeout as e:
             self.assertEqual(e.timeout, 2)
+            # Try to get the string repr. of the exception: see bug #658
+            s = str(e)
+        else:
+            self.fail('expected timeout')
 
     def test_command_async(self):
         from datetime import datetime