diff --git a/py/mpz.c b/py/mpz.c
index a6b024ca87..b42e96ee18 100644
--- a/py/mpz.c
+++ b/py/mpz.c
@@ -1255,7 +1255,7 @@ uint mpz_as_str_inpl(const mpz_t *i, uint base, const char *prefix, char base_ch
         return s - str;
     }
 
-    // make a copy of mpz digits
+    // make a copy of mpz digits, so we can do the div/mod calculation
     mpz_dig_t *dig = m_new(mpz_dig_t, ilen);
     memcpy(dig, i->dig, ilen * sizeof(mpz_dig_t));
 
@@ -1295,6 +1295,9 @@ uint mpz_as_str_inpl(const mpz_t *i, uint base, const char *prefix, char base_ch
     }
     while (!done);
 
+    // free the copy of the digits array
+    m_del(mpz_dig_t, dig, ilen);
+
     if (prefix) {
         const char *p = &prefix[strlen(prefix)];
         while (p > prefix) {