diff --git a/sim/mcuboot-sys/build.rs b/sim/mcuboot-sys/build.rs
index e6414cb..d181e34 100644
--- a/sim/mcuboot-sys/build.rs
+++ b/sim/mcuboot-sys/build.rs
@@ -51,6 +51,7 @@
 
     if overwrite_only {
         conf.define("MCUBOOT_OVERWRITE_ONLY", None);
+        conf.define("MCUBOOT_OVERWRITE_ONLY_FAST", None);
     }
 
     conf.file("../../boot/bootutil/src/image_validate.c");