diff --git a/pkgs/applications/editors/emacs/elisp-packages/melpa-packages.nix b/pkgs/applications/editors/emacs/elisp-packages/melpa-packages.nix
index 2ed31090e106..1108d0e154b6 100644
--- a/pkgs/applications/editors/emacs/elisp-packages/melpa-packages.nix
+++ b/pkgs/applications/editors/emacs/elisp-packages/melpa-packages.nix
@@ -294,7 +294,7 @@ let
 
         irony = super.irony.overrideAttrs (old: {
           cmakeFlags = old.cmakeFlags or [ ] ++ [ "-DCMAKE_INSTALL_BINDIR=bin" ];
-          env.NIX_CFLAGS_COMPILE = "-UCLANG_RESOURCE_DIR";
+          env = old.env or { } // { NIX_CFLAGS_COMPILE = "-UCLANG_RESOURCE_DIR"; };
           preConfigure = ''
             pushd server
           '';