diff --git a/functions-common b/functions-common
index 40a0d2b..30f99ba 100644
--- a/functions-common
+++ b/functions-common
@@ -26,7 +26,6 @@
 # - ``ERROR_ON_CLONE``
 # - ``FILES``
 # - ``OFFLINE``
-# - ``PIP_DOWNLOAD_CACHE``
 # - ``RECLONE``
 # - ``REQUIREMENTS_DIR``
 # - ``STACK_USER``
@@ -1555,8 +1554,7 @@
 }
 
 # Wrapper for ``pip install`` to set cache and proxy environment variables
-# Uses globals ``OFFLINE``, ``PIP_DOWNLOAD_CACHE``,
-# ``TRACK_DEPENDS``, ``*_proxy``
+# Uses globals ``OFFLINE``, ``TRACK_DEPENDS``, ``*_proxy``
 # pip_install package [package ...]
 function pip_install {
     local xtrace=$(set +o | grep xtrace)
@@ -1581,8 +1579,15 @@
         local sudo_pip="sudo -H"
     fi
 
+    local pip_version=$(python -c "import pip; \
+                        print(pip.__version__.strip('.')[0])")
+    if (( pip_version<6 )); then
+        die $LINENO "Currently installed pip version ${pip_version} does not" \
+            "meet minimum requirements (>=6)."
+    fi
+
     $xtrace
-    $sudo_pip PIP_DOWNLOAD_CACHE=${PIP_DOWNLOAD_CACHE:-/var/cache/pip} \
+    $sudo_pip \
         http_proxy=$http_proxy \
         https_proxy=$https_proxy \
         no_proxy=$no_proxy \
@@ -1593,7 +1598,7 @@
     if [[ "$INSTALL_TESTONLY_PACKAGES" == "True" ]]; then
         local test_req="$@/test-requirements.txt"
         if [[ -e "$test_req" ]]; then
-            $sudo_pip PIP_DOWNLOAD_CACHE=${PIP_DOWNLOAD_CACHE:-/var/cache/pip} \
+            $sudo_pip \
                 http_proxy=$http_proxy \
                 https_proxy=$https_proxy \
                 no_proxy=$no_proxy \