diff --git a/pkgs/top-level/python-packages.nix b/pkgs/top-level/python-packages.nix
index 3b0946a0579..7023ff68e71 100644
--- a/pkgs/top-level/python-packages.nix
+++ b/pkgs/top-level/python-packages.nix
@@ -16941,6 +16941,11 @@ EOF
       sha256 = "0v8ziaam2r637v94ra4dbjw6jzxz99gs5x4i585kgag1v204yb9b";
     };
 
+    checkPhase = ''
+      # The tests assume that test_xmlast does not run before test_pyPEG2.
+      python -m unittest pypeg2.test.test_pyPEG2 pypeg2.test.test_xmlast
+    '';
+
     #https://bitbucket.org/fdik/pypeg/issues/36/test-failures-on-py35
     doCheck = !isPy3k;