diff --git a/elements/pypi/README.md b/elements/pypi/README.md
index c32e2389..96e3085c 100644
--- a/elements/pypi/README.md
+++ b/elements/pypi/README.md
@@ -2,8 +2,9 @@ Inject a PyPI mirror
 ====================
 
 Use a custom PyPI mirror to build images. The default is to bind mount one from
-~/.cache/image-create/pypi/mirror into the build environment. The element
-temporarily overwrites /root/.pip.conf and .pydistutils.cfg to use it.
+~/.cache/image-create/pypi/mirror into the build environment as mirror URL
+file:///tmp/pypi. The element temporarily overwrites /root/.pip.conf
+and .pydistutils.cfg to use it.
 
 When online, the official pypi.python.org pypi index is supplied as an
 extra-url, so uncached dependencies will still be available. When offline, only
@@ -23,6 +24,7 @@ you have more than 9 additional mirrors, some care will be needed.
 A typical use of this element is thus:
 export PYPI\_MIRROR\_URL=http://site/pypi/Ubuntu-13.10
 export PYPI\_MIRROR\_URL\_1=http://site/pypi/
+export PYPI\_MIRROR\_URL\_2=file:///tmp/pypi
 
 [pypi-mirror](https://git.openstack.org/cgit/openstack-infra/pypi-mirror) can
 be useful in making a partial PyPI mirror suitable for building images. For
diff --git a/elements/pypi/extra-data.d/00-mount-pypi-mirror b/elements/pypi/extra-data.d/00-mount-pypi-mirror
index 709a11e8..0a3edd82 100755
--- a/elements/pypi/extra-data.d/00-mount-pypi-mirror
+++ b/elements/pypi/extra-data.d/00-mount-pypi-mirror
@@ -3,13 +3,10 @@
 set -eu
 set -o pipefail
 
-PYPI_MIRROR_URL=${PYPI_MIRROR_URL:-''}
-if [ -n "$PYPI_MIRROR_URL" ]; then
-    # External mirror URL being used - no need to bind mount anything.
-    exit 0
-fi
 MIRROR_SOURCE=~/.cache/image-create/pypi/mirror/
-MIRROR_TARGET=$TMP_MOUNT_PATH/tmp/pypi
+if [ -d "$MIRROR_SOURCE" ]; then
+    MIRROR_TARGET=$TMP_MOUNT_PATH/tmp/pypi
 
-sudo mkdir -p $MIRROR_SOURCE $MIRROR_TARGET
-sudo mount --bind $MIRROR_SOURCE $TMP_MOUNT_PATH/tmp/pypi
+    sudo mkdir -p $MIRROR_SOURCE $MIRROR_TARGET
+    sudo mount --bind $MIRROR_SOURCE $MIRROR_TARGET
+fi