--- a/testing/mozharness/mozprocess/processhandler.py
+++ b/testing/mozharness/mozprocess/processhandler.py
@@ -899,16 +899,19 @@ class ProcessHandler(ProcessHandlerMixin
 
     If logfile is not None, the output produced by the process will be
     appended to the given file.
     """
 
     def __init__(self, cmd, logfile=None, storeOutput=True, **kwargs):
         kwargs.setdefault('processOutputLine', [])
 
+        if not isinstance(kwargs['processOutputLine'], (list, tuple)):
+            kwargs['processOutputLine'] = [kwargs['processOutputLine']]
+
         # Print to standard output only if no outputline provided
         if not kwargs['processOutputLine']:
             kwargs['processOutputLine'].append(print_output)
 
         if logfile:
             logoutput = LogOutput(logfile)
             kwargs['processOutputLine'].append(logoutput)