diff --git a/test/objspace/test_objspace.rb b/test/objspace/test_objspace.rb
index 602773b6cc..833b372105 100644
--- a/test/objspace/test_objspace.rb
+++ b/test/objspace/test_objspace.rb
@@ -400,7 +400,7 @@ class TestObjSpace < Test::Unit::TestCase
         def dump_my_heap_please
           ObjectSpace.trace_object_allocations_start
           GC.start
-          str = "TEST STRING".force_encoding("UTF-8")
+          (str = "TEST STRING").force_encoding("UTF-8")
           ObjectSpace.dump_all().path
         end