diff --git a/IPython/core/alias.py b/IPython/core/alias.py
index e1a6242..3d79908 100644
--- a/IPython/core/alias.py
+++ b/IPython/core/alias.py
@@ -151,8 +151,8 @@ class Alias(object):
             raise InvalidAliasError("An alias command must be a string, "
                                     "got: %r" % self.cmd)
 
-        nargs = self.cmd.count('%s')
-
+        nargs = self.cmd.count('%s') - self.cmd.count('%%s')
+  
         if (nargs > 0) and (self.cmd.find('%l') >= 0):
             raise InvalidAliasError('The %s and %l specifiers are mutually '
                                     'exclusive in alias definitions.')