diff --git a/django/db/models/fields/__init__.py b/django/db/models/fields/__init__.py
index b7438e02b25..273416559cd 100644
--- a/django/db/models/fields/__init__.py
+++ b/django/db/models/fields/__init__.py
@@ -198,9 +198,9 @@ class Field(object):
             # be invoked before the final SQL is evaluated
             if hasattr(value, 'relabel_aliases'):
                 return value
-            try:
+            if hasattr(value, 'as_sql'):
                 sql, params = value.as_sql()
-            except AttributeError:
+            else:
                 sql, params = value._as_sql()
             return QueryWrapper(('(%s)' % sql), params)
 
diff --git a/django/db/models/fields/related.py b/django/db/models/fields/related.py
index 8f874bc7e0d..9aa2421f034 100644
--- a/django/db/models/fields/related.py
+++ b/django/db/models/fields/related.py
@@ -145,9 +145,9 @@ class RelatedField(object):
             # be invoked before the final SQL is evaluated
             if hasattr(value, 'relabel_aliases'):
                 return value
-            try:
+            if hasattr(value, 'as_sql'):
                 sql, params = value.as_sql()
-            except AttributeError:
+            else:
                 sql, params = value._as_sql()
             return QueryWrapper(('(%s)' % sql), params)