diff --git a/library/constant_time.h b/library/constant_time.h
index 0b75900..8e8992d 100644
--- a/library/constant_time.h
+++ b/library/constant_time.h
@@ -35,3 +35,5 @@
 size_t mbedtls_cf_size_mask( size_t bit );
 
 size_t mbedtls_cf_size_mask_lt( size_t x, size_t y );
+
+size_t mbedtls_cf_size_mask_ge( size_t x, size_t y );