diff --git a/uberenv.py b/uberenv.py
index 5090657..af89d9f 100755
--- a/uberenv.py
+++ b/uberenv.py
@@ -978,9 +978,9 @@ def concretize_spack_env(self):
         sexe(spack_concretize_cmd, echo=True)
 
     def clean_build(self):
-        # clean out any spack cached stuff (except build stages, downloads, &
-        # spack's bootstrapping software)
-        cln_cmd = "{0} clean --misc-cache --failures --python-cache".format(self.spack_exe(use_spack_env=False))
+        # clean out any spack cached stuff (except build stages, downloads)
+        # note: we now clean out spack bootstrap b/c ~/.spack contents can undermine bootstrap
+        cln_cmd = "{0} clean --bootstrap --misc-cache --failures --python-cache".format(self.spack_exe(use_spack_env=False))
         res = sexe(cln_cmd, echo=True)
 
         # check if we need to force uninstall of selected packages