diff --git a/pkgs/development/python-modules/objgraph/default.nix b/pkgs/development/python-modules/objgraph/default.nix
index c7e924569ba1..16dc60fc28a3 100644
--- a/pkgs/development/python-modules/objgraph/default.nix
+++ b/pkgs/development/python-modules/objgraph/default.nix
@@ -1,8 +1,9 @@
 { stdenv
 , buildPythonPackage
 , fetchPypi
-, pkgs
 , isPyPy
+, graphviz
+, mock
 }:
 
 buildPythonPackage rec {
@@ -17,7 +18,9 @@ buildPythonPackage rec {
   # Tests fail with PyPy.
   disabled = isPyPy;
 
-  propagatedBuildInputs = [pkgs.graphviz];
+  propagatedBuildInputs = [ graphviz ];
+
+  checkInputs = [ mock ];
 
   meta = with stdenv.lib; {
     description = "Draws Python object reference graphs with graphviz";