diff --git a/test/ipaddress/prefix_test.rb b/test/ipaddress/prefix_test.rb
index e6ac607..56211cf 100644
--- a/test/ipaddress/prefix_test.rb
+++ b/test/ipaddress/prefix_test.rb
@@ -97,7 +97,7 @@ def test_method_plus
   def test_method_minus
     p1 = @klass.new 8
     p2 = @klass.new 24
-    assert_equal -16, p1-p2
+    assert_equal (-16), p1-p2
     assert_equal 16, p2-p1
     assert_equal 20, p2-4
     assert_equal 20r, p2-4r