diff --git a/library/rsa.c b/library/rsa.c
index 4ee9308bd..862301190 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -923,7 +923,7 @@ void mbedtls_rsa_set_padding( mbedtls_rsa_context *ctx, int padding, int hash_id
 
 size_t mbedtls_rsa_get_len( const mbedtls_rsa_context *ctx )
 {
-    return( mbedtls_mpi_size( &ctx->N ) );
+    return( ctx->len );
 }