diff --git a/meta-arm-autonomy/conf/machine/arm64-autonomy-guest.conf b/meta-arm-autonomy/conf/machine/arm64-autonomy-guest.conf
index 097b382a..c04fc23a 100644
--- a/meta-arm-autonomy/conf/machine/arm64-autonomy-guest.conf
+++ b/meta-arm-autonomy/conf/machine/arm64-autonomy-guest.conf
@@ -8,7 +8,7 @@ require conf/machine/include/arm/arch-armv8a.inc
 
 KERNEL_IMAGETYPE = "Image"
 
-IMAGE_FSTYPES += "tar.bz2"
+IMAGE_FSTYPES:append = " tar.bz2"
 
 PREFERRED_PROVIDER_virtual/kernel = "linux-yocto"
 PREFERRED_VERSION_linux-yocto ?= "5.10%"