diff --git a/lib/spack/external/py/_path/local.py b/lib/spack/external/py/_path/local.py
index 2ffdaddf06b..d2f16b993ea 100644
--- a/lib/spack/external/py/_path/local.py
+++ b/lib/spack/external/py/_path/local.py
@@ -10,7 +10,7 @@
 from py._path.common import iswin32, fspath
 from stat import S_ISLNK, S_ISDIR, S_ISREG
 
-from os.path import abspath, normcase, normpath, isabs, exists, isdir, isfile, islink, dirname
+from os.path import abspath, normpath, isabs, exists, isdir, isfile, islink, dirname
 
 if sys.version_info > (3,0):
     def map_as_list(func, iter):
@@ -801,10 +801,10 @@ def make_numbered_dir(cls, prefix='session-', rootdir=None, keep=3,
         if rootdir is None:
             rootdir = cls.get_temproot()
 
-        nprefix = normcase(prefix)
+        nprefix = prefix.lower()
         def parse_num(path):
             """ parse the number out of a path (if it matches the prefix) """
-            nbasename = normcase(path.basename)
+            nbasename = path.basename.lower()
             if nbasename.startswith(nprefix):
                 try:
                     return int(nbasename[len(nprefix):])