diff --git a/class.c b/class.c
index 6d81654142..72817d0824 100644
--- a/class.c
+++ b/class.c
@@ -702,7 +702,7 @@ class_alloc(VALUE flags, VALUE klass)
     RCLASS_PRIME_NS((VALUE)obj) = ns;
     // Classes/Modules defined in user namespaces are
     // writable directly because it exists only in a namespace.
-    RCLASS_SET_PRIME_CLASSEXT_WRITABLE((VALUE)obj, NAMESPACE_USER_P(ns) ? true : false);
+    RCLASS_SET_PRIME_CLASSEXT_WRITABLE((VALUE)obj, (NAMESPACE_USER_P(ns) || !rb_namespace_available()) ? true : false);
 
     RCLASS_SET_ORIGIN((VALUE)obj, (VALUE)obj);
     RCLASS_SET_REFINED_CLASS((VALUE)obj, Qnil);
@@ -858,6 +858,8 @@ rb_class_new(VALUE super)
         RCLASS_SET_MAX_IV_COUNT(klass, RCLASS_MAX_IV_COUNT(super));
     }
 
+    RUBY_ASSERT(getenv("RUBY_NAMESPACE") || RCLASS_PRIME_CLASSEXT_WRITABLE_P(klass));
+
     return klass;
 }
 
diff --git a/gc.c b/gc.c
index 8cba7fbac8..c3f0a71df1 100644
--- a/gc.c
+++ b/gc.c
@@ -3279,12 +3279,14 @@ rb_gc_mark_children(void *objspace, VALUE obj)
             // Skip updating max_iv_count if the prime classext is not writable
             // because GC context doesn't provide information about namespaces.
             if (RCLASS_PRIME_CLASSEXT_WRITABLE_P(klass)) {
-                VM_ASSERT(rb_shape_obj_too_complex_p(klass));
                 // Increment max_iv_count if applicable, used to determine size pool allocation
                 if (RCLASS_MAX_IV_COUNT(klass) < fields_count) {
                     RCLASS_SET_MAX_IV_COUNT(klass, fields_count);
                 }
             }
+            else {
+                VM_ASSERT(rb_shape_obj_too_complex_p(klass));
+            }
         }
 
         break;