diff --git a/src/runtime/compact.cpp b/src/runtime/compact.cpp
index 0a4ff61895..fe4e8be044 100644
--- a/src/runtime/compact.cpp
+++ b/src/runtime/compact.cpp
@@ -274,7 +274,7 @@ void object_compactor::insert_mpz(object * o) {
     size_t data_sz = sizeof(mpn_digit) * to_mpz(o)->m_value.m_size;
     size_t sz      = sizeof(mpz_object) + data_sz;
     mpz_object * new_o = (mpz_object *)alloc(sz);
-    lean_set_non_heap_header_for_big((lean_object*)new_o, LeanMPZ, 0);
+    lean_set_non_heap_header((lean_object*)new_o, sz, LeanMPZ, 0);
     new_o->m_value.m_size   = to_mpz(o)->m_value.m_size;
     new_o->m_value.m_sign   = to_mpz(o)->m_value.m_sign;
     new_o->m_value.m_digits = reinterpret_cast<mpn_digit*>(reinterpret_cast<char*>(new_o) + sizeof(mpz_object));