diff --git a/library/rsa.c b/library/rsa.c
index de8e765..8cadcad 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -142,7 +142,7 @@
         return( POLARSSL_ERR_RSA_KEY_CHECK_FAILED );
 
     if( mpi_msb( &ctx->N ) < 128 ||
-        mpi_msb( &ctx->N ) > 4096 )
+        mpi_msb( &ctx->N ) > POLARSSL_MPI_MAX_BITS )
         return( POLARSSL_ERR_RSA_KEY_CHECK_FAILED );
 
     if( mpi_msb( &ctx->E ) < 2 ||