filter tag works now

--HG--
branch : trunk
diff --git a/jinja2/nodes.py b/jinja2/nodes.py
index b569a31..3b0ac3c 100644
--- a/jinja2/nodes.py
+++ b/jinja2/nodes.py
@@ -232,7 +232,7 @@
 
 class FilterBlock(Stmt):
     """Node for filter sections."""
-    fields = ('body', 'filters')
+    fields = ('body', 'filter')
 
 
 class Block(Stmt):
@@ -390,6 +390,8 @@
     fields = ('node', 'name', 'args', 'kwargs', 'dyn_args', 'dyn_kwargs')
 
     def as_const(self):
+        if self.node is None:
+            raise Impossible()
         filter = self.environment.filters.get(self.name)
         if filter is None or getattr(filter, 'contextfilter', False):
             raise nodes.Impossible()