Use space (' ') instead of '?' as a pattern terminator.
diff --git a/git-flow-release b/git-flow-release
index 95c695d..e127131 100644
--- a/git-flow-release
+++ b/git-flow-release
@@ -50,7 +50,7 @@
 	fi
 
 	current_branch=$(git branch | grep '^\* ' | grep -v 'no branch' | sed 's/^* //g')
-	short_names=$(echo "$release_branches" | sed "s?^$PREFIX??g")
+	short_names=$(echo "$release_branches" | sed "s ^$PREFIX  g")
 
 	# determine column width first
 	typeset -i width=0