diff --git a/sphinx/application.py b/sphinx/application.py
index 1c7a05357c6..641ce989305 100644
--- a/sphinx/application.py
+++ b/sphinx/application.py
@@ -1192,26 +1192,30 @@ def is_parallel_allowed(self, typ):
         """
         if typ == 'read':
             attrname = 'parallel_read_safe'
-            message = __("the %s extension does not declare if it is safe "
-                         "for parallel reading, assuming it isn't - please "
-                         "ask the extension author to check and make it "
-                         "explicit")
+            message_not_declared = __("the %s extension does not declare if it "
+                                      "is safe for parallel reading, assuming "
+                                      "it isn't - please ask the extension author "
+                                      "to check and make it explicit")
+            message_not_safe = __("the %s extension is not safe for parallel reading")
         elif typ == 'write':
             attrname = 'parallel_write_safe'
-            message = __("the %s extension does not declare if it is safe "
-                         "for parallel writing, assuming it isn't - please "
-                         "ask the extension author to check and make it "
-                         "explicit")
+            message_not_declared = __("the %s extension does not declare if it "
+                                      "is safe for parallel writing, assuming "
+                                      "it isn't - please ask the extension author "
+                                      "to check and make it explicit")
+            message_not_safe = __("the %s extension is not safe for parallel writing")
         else:
             raise ValueError('parallel type %s is not supported' % typ)
 
         for ext in self.extensions.values():
             allowed = getattr(ext, attrname, None)
             if allowed is None:
-                logger.warning(message, ext.name)
+                logger.warning(message_not_declared, ext.name)
                 logger.warning(__('doing serial %s'), typ)
                 return False
             elif not allowed:
+                logger.warning(message_not_safe, ext.name)
+                logger.warning(__('doing serial %s'), typ)
                 return False
 
         return True
diff --git a/tests/test_application.py b/tests/test_application.py
index f10592b51c0..a268f492f45 100644
--- a/tests/test_application.py
+++ b/tests/test_application.py
@@ -105,6 +105,8 @@ def test_add_is_parallel_allowed(app, status, warning):
 
     app.setup_extension('read_serial')
     assert app.is_parallel_allowed('read') is False
+    assert "the read_serial extension is not safe for parallel reading" in warning.getvalue()
+    warning.truncate(0)  # reset warnings
     assert app.is_parallel_allowed('write') is True
     assert warning.getvalue() == ''
     app.extensions.pop('read_serial')