diff --git a/sqlbase7_sa/base.py b/sqlbase7_sa/base.py
index 9392fa9..ae45cd6 100644
--- a/sqlbase7_sa/base.py
+++ b/sqlbase7_sa/base.py
@@ -41,9 +41,9 @@ class SQLBase7Compiler(SQLCompiler):
     def visit_select(self, select, **kwargs):
         froms = select._get_display_froms()
         whereclause = self._get_join_whereclause(froms)
-        if whereclause is not None:
+        if whereclause:
             select = select.where(whereclause)
-            
+
         kwargs['iswrapper'] = getattr(select, '_is_wrapper', False)
         return SQLCompiler.visit_select(self, select, **kwargs)
 
@@ -59,8 +59,10 @@ class SQLBase7Compiler(SQLCompiler):
         for f in froms:
             if isinstance(f, Join):
                 visit_join(f)
-                
-        return and_(*clauses)
+
+        if clauses:
+            return and_(*clauses)
+        return None
 
     def visit_ilike_op(self, binary, **kw):
         escape = binary.modifiers.get("escape", None)