diff --git a/crypto/ec/ec_asn1.c b/crypto/ec/ec_asn1.c
index 9bba26ed7edc2adc1363719ec1efdf7d41ea98b1..97c590691dd16673b8c8b4c975f1778e6c4280fe 100644
--- a/crypto/ec/ec_asn1.c
+++ b/crypto/ec/ec_asn1.c
@@ -1110,7 +1110,7 @@ int i2d_ECPrivateKey(EC_KEY *a, unsigned char **out)
 {
     int ret = 0, ok = 0;
     unsigned char *buffer = NULL;
-    size_t buf_len = 0, tmp_len;
+    size_t buf_len = 0, tmp_len, bn_len;
     EC_PRIVATEKEY *priv_key = NULL;
 
     if (a == NULL || a->group == NULL || a->priv_key == NULL ||
@@ -1126,18 +1126,32 @@ int i2d_ECPrivateKey(EC_KEY *a, unsigned char **out)
 
     priv_key->version = a->version;
 
-    buf_len = (size_t)BN_num_bytes(a->priv_key);
+    bn_len = (size_t)BN_num_bytes(a->priv_key);
+
+    /* Octetstring may need leading zeros if BN is to short */
+
+    buf_len = (EC_GROUP_get_degree(a->group) + 7) / 8;
+
+    if (bn_len > buf_len) {
+        ECerr(EC_F_I2D_ECPRIVATEKEY, EC_R_BUFFER_TOO_SMALL);
+        goto err;
+    }
+
     buffer = OPENSSL_malloc(buf_len);
     if (buffer == NULL) {
         ECerr(EC_F_I2D_ECPRIVATEKEY, ERR_R_MALLOC_FAILURE);
         goto err;
     }
 
-    if (!BN_bn2bin(a->priv_key, buffer)) {
+    if (!BN_bn2bin(a->priv_key, buffer + buf_len - bn_len)) {
         ECerr(EC_F_I2D_ECPRIVATEKEY, ERR_R_BN_LIB);
         goto err;
     }
 
+    if (buf_len - bn_len > 0) {
+        memset(buffer, 0, buf_len - bn_len);
+    }
+
     if (!ASN1_OCTET_STRING_set(priv_key->privateKey, buffer, buf_len)) {
         ECerr(EC_F_I2D_ECPRIVATEKEY, ERR_R_ASN1_LIB);
         goto err;