diff --git a/sim/mcuboot-sys/build.rs b/sim/mcuboot-sys/build.rs
index 4e5cf66..b606ab9 100644
--- a/sim/mcuboot-sys/build.rs
+++ b/sim/mcuboot-sys/build.rs
@@ -39,6 +39,7 @@
 
     if bootstrap {
         conf.define("MCUBOOT_BOOTSTRAP", None);
+        conf.define("MCUBOOT_OVERWRITE_ONLY_FAST", None);
     }
 
     if validate_primary_slot {
@@ -121,7 +122,6 @@
 
     if overwrite_only {
         conf.define("MCUBOOT_OVERWRITE_ONLY", None);
-        conf.define("MCUBOOT_OVERWRITE_ONLY_FAST", None);
     }
 
     if swap_move {