diff --git a/ractor.c b/ractor.c
index 484b5c78b9..585435c820 100644
--- a/ractor.c
+++ b/ractor.c
@@ -2445,10 +2445,10 @@ rb_ractor_make_shareable_copy(VALUE obj)
 VALUE
 rb_ractor_ensure_shareable(VALUE obj, VALUE name)
 {
-    if (!RTEST(rb_ractor_shareable_p(obj))) {
+    if (!rb_ractor_shareable_p(obj)) {
         VALUE message = rb_sprintf("cannot assign unshareable object to %"PRIsVALUE,
                                    name);
-        rb_exc_raise(rb_exc_new_str(rb_eRactorError, message));
+        rb_exc_raise(rb_exc_new_str(rb_eRactorIsolationError, message));
     }
     return obj;
 }
diff --git a/test/ruby/test_parse.rb b/test/ruby/test_parse.rb
index 8d12538fa3..aeaa42dadb 100644
--- a/test/ruby/test_parse.rb
+++ b/test/ruby/test_parse.rb
@@ -1178,8 +1178,8 @@ x = __ENCODING__
     Class.new.class_eval(code)
   end
 
-  def assert_ractor_error(message, code)
-    assert_raise_with_message(Ractor::Error, message) do
+  def assert_raise_separately(error, message, code)
+    assert_raise_with_message(error, message) do
       eval_separately(code)
     end
   end
@@ -1243,7 +1243,8 @@ x = __ENCODING__
   end
 
   def test_shareable_constant_value_unshareable_literal
-    assert_ractor_error(/unshareable/, "#{<<~"begin;"}\n#{<<~'end;'}")
+    assert_raise_separately(Ractor::IsolationError, /unshareable/,
+                            "#{<<~"begin;"}\n#{<<~'end;'}")
     begin;
       # shareable_constant_value: literal
       C = ["Not " + "shareable"]
@@ -1264,7 +1265,8 @@ x = __ENCODING__
   end
 
   def test_shareable_constant_value_unfrozen
-    assert_ractor_error(/does not freeze object correctly/, "#{<<~"begin;"}\n#{<<~'end;'}")
+    assert_raise_separately(Ractor::Error, /does not freeze object correctly/,
+                            "#{<<~"begin;"}\n#{<<~'end;'}")
     begin;
       # shareable_constant_value: experimental_everything
       o = Object.new