diff --git a/activerecord/lib/active_record/attribute_methods/primary_key.rb b/activerecord/lib/active_record/attribute_methods/primary_key.rb
index a10e94d5f703365a61efe462dd9b0ed1f9ed88d5..5ae1ebc9cf7dd1712ee1a82ea4a0c069348543b6 100644
--- a/activerecord/lib/active_record/attribute_methods/primary_key.rb
+++ b/activerecord/lib/active_record/attribute_methods/primary_key.rb
@@ -24,11 +24,13 @@ def reset_primary_key #:nodoc:
         end
 
         def get_primary_key(base_name) #:nodoc:
+          return unless base_name
+
           case primary_key_prefix_type
           when :table_name
-            base_name.to_s.foreign_key(false)
+            base_name.foreign_key(false)
           when :table_name_with_underscore
-            base_name.to_s.foreign_key
+            base_name.foreign_key
           else
             'id'
           end
diff --git a/activerecord/test/cases/base_test.rb b/activerecord/test/cases/base_test.rb
index 86d4a90fc45371464893d0edf74a9dfbc43d8776..dc5e91241244a8c9bc89acaf42f9deb77b7d649b 100644
--- a/activerecord/test/cases/base_test.rb
+++ b/activerecord/test/cases/base_test.rb
@@ -1069,6 +1069,7 @@ def test_define_attr_method_with_value
 
   def test_define_attr_method_with_block
     k = Class.new( ActiveRecord::Base )
+    k.primary_key = "id"
     k.send(:define_attr_method, :primary_key) { "sys_" + original_primary_key }
     assert_equal "sys_id", k.primary_key
   end
@@ -1109,6 +1110,7 @@ def test_set_primary_key_with_value
 
   def test_set_primary_key_with_block
     k = Class.new( ActiveRecord::Base )
+    k.primary_key = 'id'
     k.set_primary_key { "sys_" + original_primary_key }
     assert_equal "sys_id", k.primary_key
   end