diff --git a/endpoints/decorators.py b/endpoints/decorators.py
index 3e91a074c..b863a8062 100644
--- a/endpoints/decorators.py
+++ b/endpoints/decorators.py
@@ -9,7 +9,7 @@ import features
 
 from app import app
 from auth.auth_context import get_authenticated_context
-from util.names import parse_namespace_repository
+from util.names import parse_namespace_repository, ImplicitLibraryNamespaceNotAllowed
 from util.http import abort
 
 logger = logging.getLogger(__name__)
@@ -26,9 +26,14 @@ def parse_repository_name(include_tag=False,
   def inner(func):
     @wraps(func)
     def wrapper(*args, **kwargs):
-      repo_name_components = parse_namespace_repository(kwargs[incoming_repo_kwarg],
-                                                        app.config['LIBRARY_NAMESPACE'],
-                                                        include_tag=include_tag)
+      try:
+        repo_name_components = parse_namespace_repository(kwargs[incoming_repo_kwarg],
+                                                          app.config['LIBRARY_NAMESPACE'],
+                                                          include_tag=include_tag,
+                                                          allow_library=features.LIBRARY_SUPPORT)
+      except ImplicitLibraryNamespaceNotAllowed:
+        abort(400)
+
       del kwargs[incoming_repo_kwarg]
       kwargs[ns_kwarg_name] = repo_name_components[0]
       kwargs[repo_kwarg_name] = repo_name_components[1]
diff --git a/util/names.py b/util/names.py
index 9fb6f3064..e8eb72645 100644
--- a/util/names.py
+++ b/util/names.py
@@ -14,6 +14,13 @@ TAG_REGEX = re.compile(FULL_TAG_PATTERN)
 TAG_ERROR = ('Invalid tag: must match [A-Za-z0-9_.-], NOT start with "." or "-", '
              'and can contain 1-128 characters')
 
+
+class ImplicitLibraryNamespaceNotAllowed(Exception):
+  """ Exception raised if the implicit library namespace was specified but is
+      not allowed. """
+  pass
+
+
 def escape_tag(tag, default='latest'):
   """ Escapes a Docker tag, ensuring it matches the tag regular expression. """
   if not tag:
@@ -24,13 +31,16 @@ def escape_tag(tag, default='latest'):
   return tag[0:127]
 
 
-def parse_namespace_repository(repository, library_namespace, include_tag=False):
+def parse_namespace_repository(repository, library_namespace, include_tag=False,
+                               allow_library=True):
   repository = repository.encode('unidecode', 'ignore')
 
   parts = repository.rstrip('/').split('/', 1)
   if len(parts) < 2:
     namespace = library_namespace
     repository = parts[0]
+    if not allow_library:
+      raise ImplicitLibraryNamespaceNotAllowed()
   else:
     (namespace, repository) = parts