diff --git a/pkgs/by-name/pr/professor/package.nix b/pkgs/by-name/pr/professor/package.nix
index 3c9765514cb0..63221b12a055 100644
--- a/pkgs/by-name/pr/professor/package.nix
+++ b/pkgs/by-name/pr/professor/package.nix
@@ -49,8 +49,10 @@ stdenv.mkDerivation {
     yoda
   ];
 
-  CPPFLAGS = [ "-I${eigen}/include/eigen3" ];
-  PREFIX = placeholder "out";
+  env = {
+    CPPFLAGS = toString [ "-I${eigen}/include/eigen3" ];
+    PREFIX = placeholder "out";
+  };
 
   postInstall = ''
     for prog in "$out"/bin/*; do