diff --git a/gc.c b/gc.c
index edb78a1923..302b95f664 100644
--- a/gc.c
+++ b/gc.c
@@ -3192,9 +3192,14 @@ rb_gc_mark_children(void *objspace, VALUE obj)
         if (fields_count) {
             VALUE klass = RBASIC_CLASS(obj);
 
-            // Increment max_iv_count if applicable, used to determine size pool allocation
-            if (RCLASS_MAX_IV_COUNT(klass) < fields_count) {
-                RCLASS_WRITE_MAX_IV_COUNT(klass, fields_count);
+            // 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);
+                }
             }
         }