diff --git a/py/mpz.c b/py/mpz.c
index 7f2b257673..1090d58c76 100644
--- a/py/mpz.c
+++ b/py/mpz.c
@@ -1677,7 +1677,7 @@ mp_float_t mpz_as_float(const mpz_t *i) {
 #if 0
 this function is unused
 char *mpz_as_str(const mpz_t *i, mp_uint_t base) {
-    char *s = m_new(char, mpz_as_str_size(i, base, NULL, '\0'));
+    char *s = m_new(char, mp_int_format_size(mpz_max_num_bits(i), base, NULL, '\0'));
     mpz_as_str_inpl(i, base, NULL, 'a', '\0', s);
     return s;
 }
diff --git a/py/mpz.h b/py/mpz.h
index c792d87c56..1e91ca3304 100644
--- a/py/mpz.h
+++ b/py/mpz.h
@@ -137,7 +137,6 @@ void mpz_as_bytes(const mpz_t *z, bool big_endian, size_t len, byte *buf);
 #if MICROPY_PY_BUILTINS_FLOAT
 mp_float_t mpz_as_float(const mpz_t *z);
 #endif
-size_t mpz_as_str_size(const mpz_t *i, mp_uint_t base, const char *prefix, char comma);
 size_t mpz_as_str_inpl(const mpz_t *z, mp_uint_t base, const char *prefix, char base_char, char comma, char *str);
 
 #endif // __MICROPY_INCLUDED_PY_MPZ_H__