diff --git a/library/constant_time.h b/library/constant_time.h
index 32e0dfe..962af36 100644
--- a/library/constant_time.h
+++ b/library/constant_time.h
@@ -50,8 +50,8 @@
 size_t mbedtls_cf_size_mask_ge( size_t x,
                                 size_t y );
 
-size_t mbedtls_cf_size_bool_eq( size_t x,
-                                size_t y );
+unsigned mbedtls_cf_size_bool_eq( size_t x,
+                                  size_t y );
 
 unsigned mbedtls_cf_size_gt( size_t x,
                              size_t y );