diff --git a/mk-poetry-dep.nix b/mk-poetry-dep.nix
index b45a82a03..c4cf610b7 100644
--- a/mk-poetry-dep.nix
+++ b/mk-poetry-dep.nix
@@ -166,12 +166,12 @@ pythonPackages.callPackage
             builtins.fetchGit ({
               inherit (source) url;
               rev = source.resolved_reference or source.reference;
-              ref = sourceSpec.branch or sourceSpec.rev or (if sourceSpec?tag then "refs/tags/${sourceSpec.tag}" else "HEAD");
+              ref = sourceSpec.branch or (if sourceSpec ? tag then "refs/tags/${sourceSpec.tag}" else "HEAD");
             } // (
               let
                 nixVersion = builtins.substring 0 3 builtins.nixVersion;
               in
-              lib.optionalAttrs (lib.versionAtLeast nixVersion "2.4") {
+              lib.optionalAttrs ((sourceSpec ? rev) && (lib.versionAtLeast nixVersion "2.4")) {
                 allRefs = true;
               }
             ))