diff --git a/pkgs/development/ruby-modules/gem/default.nix b/pkgs/development/ruby-modules/gem/default.nix
index b8d4df1128a0..7105663413a2 100644
--- a/pkgs/development/ruby-modules/gem/default.nix
+++ b/pkgs/development/ruby-modules/gem/default.nix
@@ -136,7 +136,7 @@ stdenv.mkDerivation ((builtins.removeAttrs attrs ["source"]) // {
 
   # As of ruby 3.0, ruby headers require -fdeclspec when building with clang
   # Introduced in https://github.com/ruby/ruby/commit/0958e19ffb047781fe1506760c7cbd8d7fe74e57
-  env.NIX_CFLAGS_COMPILE = toString (lib.optionals (stdenv.cc.isClang && lib.versionAtLeast ruby.version.major "3") [
+  env.NIX_CFLAGS_COMPILE = toString (lib.optionals (ruby.rubyEngine == "ruby" && stdenv.cc.isClang && lib.versionAtLeast ruby.version.major "3") [
     "-fdeclspec"
   ]);