Const correctness
diff --git a/include/polarssl/x509_csr.h b/include/polarssl/x509_csr.h
index 30ef7c5..7e38300 100644
--- a/include/polarssl/x509_csr.h
+++ b/include/polarssl/x509_csr.h
@@ -155,7 +155,8 @@
  * \return          0 if subject name was parsed successfully, or
  *                  a specific error code
  */
-int x509write_csr_set_subject_name( x509write_csr *ctx, char *subject_name );
+int x509write_csr_set_subject_name( x509write_csr *ctx,
+                                    const char *subject_name );
 
 /**
  * \brief           Set the key for a CSR (public key will be included,