diff --git a/pkgs/development/compilers/crystal/build-package.nix b/pkgs/development/compilers/crystal/build-package.nix
index 7914b7d7771e..f4044b6a7781 100644
--- a/pkgs/development/compilers/crystal/build-package.nix
+++ b/pkgs/development/compilers/crystal/build-package.nix
@@ -108,9 +108,11 @@ stdenv.mkDerivation (
         ++ [ "runHook postConfigure" ]
       ));
 
-    CRFLAGS = lib.concatStringsSep " " defaultOptions;
+    env = {
+      CRFLAGS = lib.concatStringsSep " " defaultOptions;
 
-    PREFIX = placeholder "out";
+      PREFIX = placeholder "out";
+    };
 
     inherit enableParallelBuilding;
     strictDeps = true;