diff --git a/lib/spack/spack/build_environment.py b/lib/spack/spack/build_environment.py
index 8f3c454223b..1650d2ac136 100644
--- a/lib/spack/spack/build_environment.py
+++ b/lib/spack/spack/build_environment.py
@@ -1222,7 +1222,7 @@ def _setup_pkg_and_run(
         # objects can't be sent to the parent.
         exc_type = type(e)
         tb = e.__traceback__
-        tb_string = traceback.format_exception(exc_type, e, tb)
+        tb_string = "".join(traceback.format_exception(exc_type, e, tb))
 
         # build up some context from the offending package so we can
         # show that, too.