diff --git a/library/bignum.c b/library/bignum.c
index 06626c6..3cbeedc 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -55,7 +55,7 @@
     MBEDTLS_INTERNAL_VALIDATE(cond)
 
 /* Implementation that should never be optimized out by the compiler */
-#define mbedtls_mpi_zeroize_and_free(v, n) mbedtls_zeroize_and_free(v, ciL * n)
+#define mbedtls_mpi_zeroize_and_free(v, n) mbedtls_zeroize_and_free(v, ciL * (n))
 
 /*
  * Initialize one MPI