Index: django/template/__init__.py
===================================================================
--- django/template/__init__.py	(revision 4750)
+++ django/template/__init__.py	(working copy)
@@ -566,6 +566,8 @@
                 obj = None
             else:
                 if settings.TEMPLATE_STRING_IF_INVALID:
+                    if settings.TEMPLATE_STRING_IF_INVALID.count('%s') == 1:
+                        return settings.TEMPLATE_STRING_IF_INVALID % self.var
                     return settings.TEMPLATE_STRING_IF_INVALID
                 else:
                     obj = settings.TEMPLATE_STRING_IF_INVALID