diff --git a/pkgs/tools/networking/p2p/tahoe-lafs/default.nix b/pkgs/tools/networking/p2p/tahoe-lafs/default.nix
index 0017c13a7874..e333e6ab31fd 100644
--- a/pkgs/tools/networking/p2p/tahoe-lafs/default.nix
+++ b/pkgs/tools/networking/p2p/tahoe-lafs/default.nix
@@ -11,7 +11,9 @@ buildPythonPackage (rec {
     sha256 = "10j6s4wqqxb0x6plwvfnabxxl0k8jy1g1dfsrhpfgdi42f25dain";
   };
 
-  patchPhase = ''
+  patches = [ ./fix-test-copy-using-filecap.patch ];
+
+  postPatch = ''
     echo "forcing the use of \`setuptools' 0.6c9 rather than an unreleased version"
     for i in *setup.py
     do
diff --git a/pkgs/tools/networking/p2p/tahoe-lafs/fix-test-copy-using-filecap.patch b/pkgs/tools/networking/p2p/tahoe-lafs/fix-test-copy-using-filecap.patch
new file mode 100644
index 000000000000..9318e0fa204d
--- /dev/null
+++ b/pkgs/tools/networking/p2p/tahoe-lafs/fix-test-copy-using-filecap.patch
@@ -0,0 +1,23 @@
+From http://allmydata.org/trac/tahoe/ticket/936 .
+
+Wed Feb  3 23:13:37 GMT Standard Time 2010  david-sarah@jacaranda.org
+  * Fix race condition in allmydata.test.test_cli.Cp.test_copy_using_filecap
+diff -rN -u old-tahoe/src/allmydata/test/test_cli.py new-tahoe/src/allmydata/test/test_cli.py
+--- old-tahoe/src/allmydata/test/test_cli.py	2010-02-03 23:20:06.803000000 +0000
++++ new-tahoe/src/allmydata/test/test_cli.py	2010-02-03 23:20:10.483000000 +0000
+@@ -1007,13 +1007,13 @@
+         self.set_up_grid()
+         outdir = os.path.join(self.basedir, "outdir")
+         os.mkdir(outdir)
+-        self.do_cli("create-alias", "tahoe")
+         fn1 = os.path.join(self.basedir, "Metallica")
+         fn2 = os.path.join(outdir, "Not Metallica")
+         fn3 = os.path.join(outdir, "test2")
+         DATA1 = "puppies" * 10000
+         open(fn1, "wb").write(DATA1)
+-        d = self.do_cli("put", fn1)
++        d = self.do_cli("create-alias", "tahoe")
++        d.addCallback(lambda res: self.do_cli("put", fn1))
+         def _put_file((rc, out, err)):
+             self.failUnlessEqual(rc, 0)
+             # keep track of the filecap