diff --git a/docs/db-api.txt b/docs/db-api.txt
index adca8b4d5c..518e170407 100644
--- a/docs/db-api.txt
+++ b/docs/db-api.txt
@@ -1371,6 +1371,11 @@ equivalent::
     Entry.objects.filter(blog__id=3) # __exact is implied
     Entry.objects.filter(blog__pk=3) # __pk implies __id__exact
 
+.. note::
+    Because of this shortcut, you cannot have a field called ``pk`` that is not
+    the primary key of the model. It will always be replaced by the name of the
+    model's primary key in queries.
+
 Lookups that span relationships
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~