diff --git a/teuthology/exceptions.py b/teuthology/exceptions.py
index a33cec0415..7eb84b2771 100644
--- a/teuthology/exceptions.py
+++ b/teuthology/exceptions.py
@@ -57,6 +57,11 @@ def __init__(self, command, exitstatus, node=None, label=None):
         self.label = label
 
     def __str__(self):
+        if isinstance(self.command, (list, tuple)):
+            self.command = ' '.join(self.command)
+        elif not isinstance(self.command, str):
+            raise RuntimeError('variable "command" is not str, list or tuple')
+
         prefix = "Command failed"
         if self.label:
             prefix += " ({label})".format(label=self.label)