diff --git a/library/bignum.c b/library/bignum.c
index 61353ca..bb2d44a 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -386,7 +386,7 @@
 
 /* Convert x to a sign, i.e. to 1, if x is positive, or -1, if x is negative.
  * This looks awkward but generates smaller code than (x < 0 ? -1 : 1) */
-#define TO_SIGN(x) ((((mbedtls_mpi_uint) x) >> (biL - 1)) * -2 + 1)
+#define TO_SIGN(x) ((mbedtls_mpi_sint) (((mbedtls_mpi_uint) x) >> (biL - 1)) * -2 + 1)
 
 /*
  * Set value from integer