diff --git a/README.md b/README.md
index bca6048..a81bcd1 100644
--- a/README.md
+++ b/README.md
@@ -1068,6 +1068,7 @@ dirname() {
     # Usage: dirname "path"
     dir=${1%%/}
 
+    [[ $dir ]] || dir=//
     [[ $dir == */* ]] || dir=.
 
     printf '%s\n' "${dir%/*}"
diff --git a/manuscript/chapter5.txt b/manuscript/chapter5.txt
index 7bf7554..5534433 100644
--- a/manuscript/chapter5.txt
+++ b/manuscript/chapter5.txt
@@ -11,6 +11,7 @@ dirname() {
     # Usage: dirname "path"
     dir=${1%%/}
 
+    [[ $dir ]] || dir=//
     [[ $dir == */* ]] || dir=.
 
     printf '%s\n' "${dir%/*}"