diff --git a/Autobuild/precise-amd64.sh b/Autobuild/precise-amd64.sh
index cbafff37..cb5b4964 100644
--- a/Autobuild/precise-amd64.sh
+++ b/Autobuild/precise-amd64.sh
@@ -1,2 +1,2 @@
-AUTOBUILD_CONFIGURE_EXTRA="${AUTOBUILD_CONFIGURE_EXTRA} --arch=x86_64"
+AUTOBUILD_CONFIGURE_EXTRA="${AUTOBUILD_CONFIGURE_EXTRA:-_} --arch=x86_64"
 source Autobuild/debian.sh
diff --git a/Autobuild/precise-i386.sh b/Autobuild/precise-i386.sh
index e4f6c115..0de4e380 100644
--- a/Autobuild/precise-i386.sh
+++ b/Autobuild/precise-i386.sh
@@ -1,2 +1,2 @@
-AUTOBUILD_CONFIGURE_EXTRA="${AUTOBUILD_CONFIGURE_EXTRA} --arch=i686"
+AUTOBUILD_CONFIGURE_EXTRA="${AUTOBUILD_CONFIGURE_EXTRA:-_} --arch=i686"
 source Autobuild/debian.sh