diff --git a/bootstraptest/test_ractor.rb b/bootstraptest/test_ractor.rb
index 6b35bbb46b..2b268e9966 100644
--- a/bootstraptest/test_ractor.rb
+++ b/bootstraptest/test_ractor.rb
@@ -1022,7 +1022,7 @@ assert_equal '1234', %q{
 }
 
 # cvar in shareable-objects are not allowed to access from non-main Ractor
-assert_equal 'can not access class variables from non-main Ractors', %q{
+assert_equal 'can not access class variables from non-main Ractors (@@cv from C)', %q{
   class C
     @@cv = 'str'
   end
@@ -1041,7 +1041,7 @@ assert_equal 'can not access class variables from non-main Ractors', %q{
 }
 
 # also cached cvar in shareable-objects are not allowed to access from non-main Ractor
-assert_equal 'can not access class variables from non-main Ractors', %q{
+assert_equal 'can not access class variables from non-main Ractors (@@cv from C)', %q{
   class C
     @@cv = 'str'
     def self.cv
diff --git a/variable.c b/variable.c
index 47b5218566..a189927472 100644
--- a/variable.c
+++ b/variable.c
@@ -1195,10 +1195,13 @@ IVAR_ACCESSOR_SHOULD_BE_MAIN_RACTOR(ID id)
     }
 }
 
-#define CVAR_ACCESSOR_SHOULD_BE_MAIN_RACTOR() \
-  if (UNLIKELY(!rb_ractor_main_p())) { \
-      rb_raise(rb_eRactorIsolationError, "can not access class variables from non-main Ractors"); \
-  }
+static void
+CVAR_ACCESSOR_SHOULD_BE_MAIN_RACTOR(VALUE klass, ID id)
+{
+    if (UNLIKELY(!rb_ractor_main_p())) {
+        rb_raise(rb_eRactorIsolationError, "can not access class variables from non-main Ractors (%"PRIsVALUE" from %"PRIsVALUE")", rb_id2str(id), klass);
+    }
+}
 
 static inline void
 ivar_ractor_check(VALUE obj, ID id)
@@ -4202,7 +4205,7 @@ cvar_overtaken(VALUE front, VALUE target, ID id)
     }
 
 #define CVAR_LOOKUP(v,r) do {\
-    CVAR_ACCESSOR_SHOULD_BE_MAIN_RACTOR(); \
+    CVAR_ACCESSOR_SHOULD_BE_MAIN_RACTOR(klass, id); \
     if (cvar_lookup_at(klass, id, (v))) {r;}\
     CVAR_FOREACH_ANCESTORS(klass, v, r);\
 } while(0)