diff --git a/library/x509write_csr.c b/library/x509write_csr.c
index 0d62d1d485..accb44827b 100644
--- a/library/x509write_csr.c
+++ b/library/x509write_csr.c
@@ -274,17 +274,16 @@ int mbedtls_x509write_csr_pem( mbedtls_x509write_csr *ctx, unsigned char *buf, s
                        void *p_rng )
 {
     int ret;
-    unsigned char output_buf[4096];
     size_t olen = 0;
 
-    if( ( ret = mbedtls_x509write_csr_der( ctx, output_buf, sizeof(output_buf),
+    if( ( ret = mbedtls_x509write_csr_der( ctx, buf, size,
                                    f_rng, p_rng ) ) < 0 )
     {
         return( ret );
     }
 
     if( ( ret = mbedtls_pem_write_buffer( PEM_BEGIN_CSR, PEM_END_CSR,
-                                  output_buf + sizeof(output_buf) - ret,
+                                  buf + size - ret,
                                   ret, buf, size, &olen ) ) != 0 )
     {
         return( ret );