diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index 4c019ee..44b6b17 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -481,7 +481,7 @@
      */
     MBEDTLS_ASN1_CHK_ADD(pub_len,
                          mbedtls_pk_write_pubkey_der(ctx->subject_key,
-                                                     buf, c - buf));
+                                                     buf, (size_t) (c - buf)));
     c -= pub_len;
     len += pub_len;