diff --git a/library/bignum.c b/library/bignum.c
index e47e25917..a7e3fa3de 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1931,7 +1931,7 @@ static void mpi_montmul( mbedtls_mpi *A, const mbedtls_mpi *B, const mbedtls_mpi
         mpi_mul_hlp( m, B->p, d, u0 );
         mpi_mul_hlp( n, N->p, d, u1 );
 
-        *d++ = u0; d[n + 1] = 0;
+        d++; d[n + 1] = 0;
     }
 
     /* At this point, d is either the desired result or the desired result