diff --git a/pkgs/by-name/po/poco/package.nix b/pkgs/by-name/po/poco/package.nix
index 3d6dd586095f..fd6eb26914f9 100644
--- a/pkgs/by-name/po/poco/package.nix
+++ b/pkgs/by-name/po/poco/package.nix
@@ -52,8 +52,10 @@ stdenv.mkDerivation rec {
     "dev"
   ];
 
-  MYSQL_DIR = libmysqlclient;
-  MYSQL_INCLUDE_DIR = "${MYSQL_DIR}/include/mysql";
+  env = {
+    MYSQL_DIR = libmysqlclient;
+    MYSQL_INCLUDE_DIR = "${env.MYSQL_DIR}/include/mysql";
+  };
 
   cmakeFlags =
     let