diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index c3db3c4..86b4034 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -156,6 +156,7 @@
                                         0, buf + sizeof(buf) - len, len );
 }
 
+#if defined(POLARSSL_SHA1_C)
 int x509write_crt_set_subject_key_identifier( x509write_cert *ctx )
 {
     int ret;
@@ -202,6 +203,7 @@
                                    OID_SIZE( OID_AUTHORITY_KEY_IDENTIFIER ),
                                    0, buf + sizeof(buf) - len, len );
 }
+#endif /* POLARSSL_SHA1_C */
 
 int x509write_crt_set_key_usage( x509write_cert *ctx, unsigned char key_usage )
 {