diff --git a/contrib/completion/git-prompt.sh b/contrib/completion/git-prompt.sh
index 3826f52..5330e76 100644
--- a/contrib/completion/git-prompt.sh
+++ b/contrib/completion/git-prompt.sh
@@ -141,7 +141,7 @@
 
 	# parse configuration values
 	local option
-	for option in ${GIT_PS1_SHOWUPSTREAM}; do
+	for option in ${GIT_PS1_SHOWUPSTREAM-}; do
 		case "$option" in
 		git|svn) upstream_type="$option" ;;
 		verbose) verbose=1 ;;