diff --git a/py/private/run.tmpl.sh b/py/private/run.tmpl.sh
index f13eb2ca4..ec7df096a 100644
--- a/py/private/run.tmpl.sh
+++ b/py/private/run.tmpl.sh
@@ -35,7 +35,7 @@ function python_location {
 }
 
 VENV_TOOL="$(rlocation {{VENV_TOOL}})"
-VIRTUAL_ENV="$(alocation "${RUNFILES_DIR}/{{ARG_VENV_NAME}}")"
+VIRTUAL_ENV="$(alocation "${RUNFILES_DIR:?'RUNFILES_DIR' must be set}/{{ARG_VENV_NAME}}")"
 export VIRTUAL_ENV
 
 "${VENV_TOOL}" \
@@ -58,4 +58,4 @@ if [ -n "${BASH:-}" -o -n "${ZSH_VERSION:-}" ] ; then
     hash -r 2> /dev/null
 fi
 
-exec "{{EXEC_PYTHON_BIN}}" {{INTERPRETER_FLAGS}} "$(rlocation {{ENTRYPOINT}})" "$@"
\ No newline at end of file
+exec "{{EXEC_PYTHON_BIN}}" {{INTERPRETER_FLAGS}} "$(rlocation {{ENTRYPOINT}})" "$@"