diff --git a/src/allmydata/storage/common.py b/src/allmydata/storage/common.py
index 48fc77840..17a3f41b7 100644
--- a/src/allmydata/storage/common.py
+++ b/src/allmydata/storage/common.py
@@ -21,6 +21,12 @@ class UnknownContainerVersionError(Exception):
         self.filename = filename
         self.version = version
 
+    def __str__(self):
+        return "sharefile {!r} had unexpected version {!r}".format(
+            self.filename,
+            self.version,
+        )
+
 class UnknownMutableContainerVersionError(UnknownContainerVersionError):
     pass
 
diff --git a/src/allmydata/test/test_storage.py b/src/allmydata/test/test_storage.py
index 655395042..ba3d3598f 100644
--- a/src/allmydata/test/test_storage.py
+++ b/src/allmydata/test/test_storage.py
@@ -651,6 +651,7 @@ class Server(unittest.TestCase):
                                   ss.remote_get_buckets, b"si1")
         self.assertEqual(e.filename, fn)
         self.assertEqual(e.version, 0)
+        self.assertIn("had unexpected version 0", str(e))
 
     def test_disconnect(self):
         # simulate a disconnection
@@ -1136,6 +1137,8 @@ class MutableServer(unittest.TestCase):
                                   read, b"si1", [0], [(0,10)])
         self.assertEqual(e.filename, fn)
         self.assertTrue(e.version.startswith(b"BAD MAGIC"))
+        self.assertIn("had unexpected version", str(e))
+        self.assertIn("BAD MAGIC", str(e))
 
     def test_container_size(self):
         ss = self.create("test_container_size")