diff --git a/pkgs/tools/backup/s3ql/default.nix b/pkgs/tools/backup/s3ql/default.nix
index 9b2adc1d39f9..a90738d39a4c 100644
--- a/pkgs/tools/backup/s3ql/default.nix
+++ b/pkgs/tools/backup/s3ql/default.nix
@@ -1,17 +1,29 @@
-{ stdenv, fetchurl, python3Packages, sqlite  }:
+{ stdenv, fetchurl, python3Packages, sqlite, which }:
 
 python3Packages.buildPythonApplication rec {
   name = "${pname}-${version}";
   pname = "s3ql";
-  version = "2.17.1";
+  version = "2.21";
 
   src = fetchurl {
     url = "https://bitbucket.org/nikratio/${pname}/downloads/${name}.tar.bz2";
-    sha256 = "049vpvvkyia7v4v97rg2l01n43shrdxc1ik38bmjb2q4fvsh1pgx";
+    sha256 = "1mifmxbsxc2rcrydk2vs5cjfd5r0510q5y7rmavlzi8grpcqdf3d";
   };
 
-  propagatedBuildInputs = with python3Packages;
-    [ sqlite apsw pycrypto requests2 defusedxml dugong llfuse ];
+  buildInputs = [ which ]; # tests will fail without which
+  propagatedBuildInputs = with python3Packages; [
+    sqlite apsw pycrypto requests2 defusedxml dugong llfuse
+    cython pytest pytest-catchlog
+  ];
+
+  preBuild = ''
+    # https://bitbucket.org/nikratio/s3ql/issues/118/no-module-named-s3qldeltadump-running#comment-16951851
+    ${python3Packages.python.interpreter} ./setup.py build_cython build_ext --inplace
+  '';
+
+  checkPhase = ''
+    pytest tests
+  '';
 
   meta = with stdenv.lib; {
     description = "A full-featured file system for online data storage";