diff --git a/pkgs/development/compilers/unison/default.nix b/pkgs/development/compilers/unison/default.nix
index 0fd99ac0c375..cd0b7620a607 100644
--- a/pkgs/development/compilers/unison/default.nix
+++ b/pkgs/development/compilers/unison/default.nix
@@ -4,19 +4,19 @@
 , less
 }:
 
-stdenv.mkDerivation rec {
+stdenv.mkDerivation (finalAttrs: {
   pname = "unison-code-manager";
   milestone_id = "M4i";
-  version = "1.0.${milestone_id}-alpha";
+  version = "1.0.${finalAttrs.milestone_id}-alpha";
 
   src = if (stdenv.isDarwin) then
     fetchurl {
-      url = "https://github.com/unisonweb/unison/releases/download/release/${milestone_id}/ucm-macos.tar.gz";
+      url = "https://github.com/unisonweb/unison/releases/download/release/${finalAttrs.milestone_id}/ucm-macos.tar.gz";
       hash = "sha256-1Qp1SB5rCsVimZzRo1NOX8HBoMEGlIycJPm3zGTUuOw=";
     }
   else
     fetchurl {
-      url = "https://github.com/unisonweb/unison/releases/download/release/${milestone_id}/ucm-linux.tar.gz";
+      url = "https://github.com/unisonweb/unison/releases/download/release/${finalAttrs.milestone_id}/ucm-linux.tar.gz";
       hash = "sha256-Qx8vO/Vaz0VdCGXwIwRQIuMlp44hxCroQ7m7Y+m7aXk=";
     };
 
@@ -45,4 +45,4 @@ stdenv.mkDerivation rec {
     platforms = [ "x86_64-darwin" "x86_64-linux" "aarch64-darwin" ];
     mainProgram = "ucm";
   };
-}
+})