diff --git a/pkgs/development/libraries/opencv/3.x.nix b/pkgs/development/libraries/opencv/3.x.nix
index 11872cd47cc..a65e978f981 100644
--- a/pkgs/development/libraries/opencv/3.x.nix
+++ b/pkgs/development/libraries/opencv/3.x.nix
@@ -18,7 +18,7 @@
 
 , enableUnfree    ? false
 , enableIpp       ? false
-, enablePython    ? false, pythonPackages
+, enablePython    ? false, pythonPackages ? null
 , enableGtk2      ? false, gtk2
 , enableGtk3      ? false, gtk3
 , enableVtk       ? false, vtk
@@ -36,6 +36,8 @@
 
 assert blas.implementation == "openblas" && lapack.implementation == "openblas";
 
+assert enablePython -> pythonPackages != null;
+
 let
   version = "3.4.8";