diff --git a/mininet/net.py b/mininet/net.py
index 4e98c60..3b3b1ff 100755
--- a/mininet/net.py
+++ b/mininet/net.py
@@ -617,7 +617,7 @@ class Mininet( object ):
 
     # XXX This should be cleaned up
 
-    def iperf( self, hosts=None, l4Type='TCP', udpBw='10M', format='M' ):
+    def iperf( self, hosts=None, l4Type='TCP', udpBw='10M', format=None ):
         """Run iperf between two hosts.
            hosts: list of hosts; if None, uses opposite hosts
            l4Type: string, one of [ TCP, UDP ]
@@ -640,8 +640,8 @@ class Mininet( object ):
             bwArgs = '-b ' + udpBw + ' '
         elif l4Type != 'TCP':
             raise Exception( 'Unexpected l4 type: %s' % l4Type )
-        if not format == 'M':
-          iperfArgs += '-f %s ' %format
+        if format:
+            iperfArgs += '-f %s ' % format
         server.sendCmd( iperfArgs + '-s', printPid=True )
         servout = ''
         while server.lastPid is None:
diff --git a/mininet/test/test_hifi.py b/mininet/test/test_hifi.py
index 107138f..3f89b87 100755
--- a/mininet/test/test_hifi.py
+++ b/mininet/test/test_hifi.py
@@ -105,7 +105,7 @@ class testOptionsTopoCommon( object ):
         # pylint: enable-msg=W0612
         for rttval in [rttmin, rttavg, rttmax]:
             # Multiply delay by 4 to cover there & back on two links
-            self.assertWithinTolerance( rttval, DELAY_MS * 4.0, 
+            self.assertWithinTolerance( rttval, DELAY_MS * 4.0,
                                         DELAY_TOLERANCE)