diff --git a/src/allmydata/storage/common.py b/src/allmydata/storage/common.py
index e5563647f..48fc77840 100644
--- a/src/allmydata/storage/common.py
+++ b/src/allmydata/storage/common.py
@@ -16,11 +16,16 @@ from allmydata.util import base32
 # Backwards compatibility.
 from allmydata.interfaces import DataTooLargeError  # noqa: F401
 
-class UnknownMutableContainerVersionError(Exception):
-    pass
-class UnknownImmutableContainerVersionError(Exception):
+class UnknownContainerVersionError(Exception):
+    def __init__(self, filename, version):
+        self.filename = filename
+        self.version = version
+
+class UnknownMutableContainerVersionError(UnknownContainerVersionError):
     pass
 
+class UnknownImmutableContainerVersionError(UnknownContainerVersionError):
+    pass
 
 def si_b2a(storageindex):
     return base32.b2a(storageindex)
diff --git a/src/allmydata/storage/immutable.py b/src/allmydata/storage/immutable.py
index a43860138..fcc60509c 100644
--- a/src/allmydata/storage/immutable.py
+++ b/src/allmydata/storage/immutable.py
@@ -174,9 +174,7 @@ class ShareFile(object):
                 filesize = os.path.getsize(self.home)
                 (version, unused, num_leases) = struct.unpack(">LLL", f.read(0xc))
             if version != 1:
-                msg = "sharefile %s had version %d but we wanted 1" % \
-                      (filename, version)
-                raise UnknownImmutableContainerVersionError(msg)
+                raise UnknownImmutableContainerVersionError(filename, version)
             self._num_leases = num_leases
             self._lease_offset = filesize - (num_leases * self.LEASE_SIZE)
         self._data_offset = 0xc
diff --git a/src/allmydata/storage/mutable.py b/src/allmydata/storage/mutable.py
index 4abf22064..ce9cc5ff4 100644
--- a/src/allmydata/storage/mutable.py
+++ b/src/allmydata/storage/mutable.py
@@ -95,9 +95,7 @@ class MutableShareFile(object):
              data_length, extra_least_offset) = \
              struct.unpack(">32s20s32sQQ", data)
             if not self.is_valid_header(data):
-                msg = "sharefile %s had magic '%r' but we wanted '%r'" % \
-                      (filename, magic, self.MAGIC)
-                raise UnknownMutableContainerVersionError(msg)
+                raise UnknownMutableContainerVersionError(filename, magic)
         self.parent = parent # for logging
 
     def log(self, *args, **kwargs):
diff --git a/src/allmydata/test/test_storage.py b/src/allmydata/test/test_storage.py
index 2c8d84b9e..bf9eff37a 100644
--- a/src/allmydata/test/test_storage.py
+++ b/src/allmydata/test/test_storage.py
@@ -646,7 +646,8 @@ class Server(unittest.TestCase):
 
         e = self.failUnlessRaises(UnknownImmutableContainerVersionError,
                                   ss.remote_get_buckets, b"si1")
-        self.failUnlessIn(" had version 0 but we wanted 1", str(e))
+        self.assertEqual(e.filename, fn)
+        self.assertEqual(e.version, 0)
 
     def test_disconnect(self):
         # simulate a disconnection
@@ -1127,8 +1128,8 @@ class MutableServer(unittest.TestCase):
         read = ss.remote_slot_readv
         e = self.failUnlessRaises(UnknownMutableContainerVersionError,
                                   read, b"si1", [0], [(0,10)])
-        self.failUnlessIn(" had magic ", str(e))
-        self.failUnlessIn(" but we wanted ", str(e))
+        self.assertEqual(e.filename, fn)
+        self.assertTrue(e.version.startswith(b"BAD MAGIC"))
 
     def test_container_size(self):
         ss = self.create("test_container_size")