diff --git a/lib/spack/spack/binary_distribution.py b/lib/spack/spack/binary_distribution.py
index 327e6ab434a..5ce7b624522 100644
--- a/lib/spack/spack/binary_distribution.py
+++ b/lib/spack/spack/binary_distribution.py
@@ -1343,7 +1343,7 @@ def _build_tarball_in_stage_dir(
     spec_dict["buildinfo"] = buildinfo
 
     with open(specfile_path, "w") as outfile:
-        outfile.write(json.dumps(spec_dict))
+        outfile.write(sjson.dump(spec_dict))
 
     # sign the tarball and spec file with gpg
     if not unsigned: