diff --git a/bash_completion.sh.in b/bash_completion.sh.in
index 36ccd3f9..14be4e66 100644
--- a/bash_completion.sh.in
+++ b/bash_completion.sh.in
@@ -1,5 +1,5 @@
 # Check for interactive bash and that we haven't already been sourced.
-if [ -n "$BASH_VERSION" -a -n "$PS1" -a -z "$BASH_COMPLETION_COMPAT_DIR" ]; then
+if [ -n "${BASH_VERSION-}" -a -n "${PS1-}" -a -z "${BASH_COMPLETION_COMPAT_DIR-}" ]; then
 
     # Check for recent enough version of bash.
     if [ ${BASH_VERSINFO[0]} -gt 4 ] || \