diff --git a/numpy/dual.py b/numpy/dual.py
index 3a16a8ec517f..651e845bbd5c 100644
--- a/numpy/dual.py
+++ b/numpy/dual.py
@@ -51,14 +51,14 @@
 
 def register_func(name, func):
     if name not in __all__:
-        raise ValueError("%s not a dual function." % name)
+        raise ValueError("{} not a dual function.".format(name))
     f = sys._getframe(0).f_globals
     _restore_dict[name] = f[name]
     f[name] = func
 
 def restore_func(name):
     if name not in __all__:
-        raise ValueError("%s not a dual function." % name)
+        raise ValueError("{} not a dual function.".format(name))
     try:
         val = _restore_dict[name]
     except KeyError: