diff --git a/library/constant_time.h b/library/constant_time.h
index 973e856..f97c57e 100644
--- a/library/constant_time.h
+++ b/library/constant_time.h
@@ -55,3 +55,4 @@
 
 unsigned mbedtls_cf_uint_if( unsigned cond, unsigned if1, unsigned if0 );
 
+int mbedtls_cf_cond_select_sign( int a, int b, unsigned char second );