diff --git a/Lib/site.py b/Lib/site.py
--- a/Lib/site.py
+++ b/Lib/site.py
@@ -262,7 +266,7 @@ def addusersitepackages(known_paths):
     # this call will also make sure USER_BASE and USER_SITE are set
     user_site = getusersitepackages()
 
-    if ENABLE_USER_SITE and os.path.isdir(user_site):
+    if ENABLE_USER_SITE and user_site and os.path.isdir(user_site):
         addsitedir(user_site, known_paths)
     return known_paths
 
diff --git a/Lib/sysconfig.py b/Lib/sysconfig.py
--- a/Lib/sysconfig.py
+++ b/Lib/sysconfig.py
@@ -170,7 +170,13 @@ def _expand_vars(scheme, vars):
     for key, value in _INSTALL_SCHEMES[scheme].items():
         if os.name in ('posix', 'nt'):
             value = os.path.expanduser(value)
-        res[key] = os.path.normpath(_subst_vars(value, vars))
+        try:
+            path = _subst_vars(value, vars)
+        except AttributeError:
+            # a substitution variable doesn't exist (eg. '{userbase}'),
+            # don't create the variable
+            continue
+        res[key] = os.path.normpath(path)
     return res
 
 def _get_default_scheme():
@@ -488,7 +494,12 @@ def get_config_vars(*args):
         # Setting 'userbase' is done below the call to the
         # init function to enable using 'get_config_var' in
         # the init-function.
-        _CONFIG_VARS['userbase'] = _getuserbase()
+        try:
+            _CONFIG_VARS['userbase'] = _getuserbase()
+        except KeyError:
+            # catch getpwuid() error: don't create the variable if the user
+            # doesn't exist
+            pass
 
         if 'srcdir' not in _CONFIG_VARS:
             _CONFIG_VARS['srcdir'] = _PROJECT_BASE