1 files changed, 9 insertions, 0 deletions
diff --git a/lib/sqlalchemy/sql/base.py b/lib/sqlalchemy/sql/base.py
index 81685dfe0..d9f05e823 100644
--- a/lib/sqlalchemy/sql/base.py
+++ b/lib/sqlalchemy/sql/base.py
@@ -54,6 +54,8 @@ class Immutable(object):
 
 
 class SingletonConstant(Immutable):
+    """Represent SQL constants like NULL, TRUE, FALSE"""
+
     def __new__(cls, *arg, **kw):
         return cls._singleton
 
@@ -63,6 +65,13 @@ class SingletonConstant(Immutable):
         obj.__init__()
         cls._singleton = obj
 
+    # don't proxy singletons.   this means that a SingletonConstant
+    # will never be a "corresponding column" in a statement; the constant
+    # can be named directly and as it is often/usually compared against using
+    # "IS", it can't be adapted to a subquery column in any case.
+    # see :ticket:`6259`.
+    proxy_set = frozenset()
+
 
 def _from_objects(*elements):
     return itertools.chain.from_iterable(