diff --git a/library/constant_time.h b/library/constant_time.h
index ed6ec6a..80e59da 100644
--- a/library/constant_time.h
+++ b/library/constant_time.h
@@ -36,7 +36,7 @@
 
 unsigned mbedtls_cf_uint_mask( unsigned value );
 
-size_t mbedtls_cf_size_mask( size_t bit );
+size_t mbedtls_cf_size_mask( size_t value );
 
 size_t mbedtls_cf_size_mask_lt( size_t x,
                                 size_t y );