diff --git a/library/bignum_mod.h b/library/bignum_mod.h
index c25eb87423..3b3338c2de 100644
--- a/library/bignum_mod.h
+++ b/library/bignum_mod.h
@@ -53,7 +53,11 @@ typedef struct
     size_t limbs;
 } mbedtls_mpi_mod_residue;
 
-typedef void *mbedtls_mpi_mont_struct;
+typedef struct {
+    mbedtls_mpi_uint const *rr;  /* The residue for 2^{2*n*biL} mod N */
+    mbedtls_mpi_uint mm;         /* Montgomery const for -N^{-1} mod 2^{ciL} */
+} mbedtls_mpi_mont_struct;
+
 typedef void *mbedtls_mpi_opt_red_struct;
 
 typedef struct {