diff --git a/libmproxy/flow.py b/libmproxy/flow.py
index 44bb10186..8e4c2117f 100644
--- a/libmproxy/flow.py
+++ b/libmproxy/flow.py
@@ -1092,7 +1092,14 @@ class Flow:
         """
             Match this flow against a compiled filter expression. Returns True
             if matched, False if not.
+
+            If f is a string, it will be compiled as a filter expression. If
+            the expression is invalid, ValueError is raised.
         """
+        if isinstance(f, basestring):
+            f = filt.parse(f)
+            if not f:
+                raise ValueError("Invalid filter expression.")
         if f:
             return f(self)
         return True
diff --git a/test/test_flow.py b/test/test_flow.py
index 53d92f259..277d2407a 100644
--- a/test/test_flow.py
+++ b/test/test_flow.py
@@ -171,12 +171,15 @@ class TestFlow:
         f = tutils.tflow()
         f.response = tutils.tresp()
         f.request = f.response.request
-        assert not f.match(filt.parse("~b test"))
+        assert not f.match("~b test")
         assert f.match(None)
-        assert not f.match(filt.parse("~b test"))
+        assert not f.match("~b test")
 
         f = tutils.tflow_err()
-        assert f.match(filt.parse("~e"))
+        assert f.match("~e")
+
+        tutils.raises(ValueError, f.match, "~")
+
 
     def test_backup(self):
         f = tutils.tflow()