diff --git a/lib/common-functions b/lib/common-functions
index f80d5f42..22a2de11 100644
--- a/lib/common-functions
+++ b/lib/common-functions
@@ -24,8 +24,9 @@ function tmpfs_check() {
 }
 
 function mk_build_dir () {
-  export TMP_BUILD_DIR=$(mktemp -t -d --tmpdir=${TMP_DIR:-/tmp} image.XXXXXXXX)
+  TMP_BUILD_DIR=$(mktemp -t -d --tmpdir=${TMP_DIR:-/tmp} image.XXXXXXXX)
   [ $? -eq 0 ] || die "Failed to create tmp directory"
+  export TMP_BUILD_DIR
   if tmpfs_check ; then
     sudo mount -t tmpfs tmpfs $TMP_BUILD_DIR
   fi