diff --git a/README.md b/README.md
index a81bcd1..c22b3f5 100644
--- a/README.md
+++ b/README.md
@@ -1067,11 +1067,11 @@ Alternative to the `dirname` command.
 dirname() {
     # Usage: dirname "path"
     dir=${1%%/}
+    dir=${dir%/*}
 
-    [[ $dir ]] || dir=//
-    [[ $dir == */* ]] || dir=.
+    [[ $1 == */* ]] || dir=.
 
-    printf '%s\n' "${dir%/*}"
+    printf '%s\n' "${dir:-/}"
 }
 ```
 
diff --git a/manuscript/chapter5.txt b/manuscript/chapter5.txt
index 5534433..84b9e2b 100644
--- a/manuscript/chapter5.txt
+++ b/manuscript/chapter5.txt
@@ -10,11 +10,11 @@ Alternative to the `dirname` command.
 dirname() {
     # Usage: dirname "path"
     dir=${1%%/}
+    dir=${dir%/*}
 
-    [[ $dir ]] || dir=//
-    [[ $dir == */* ]] || dir=.
+    [[ $1 == */* ]] || dir=.
 
-    printf '%s\n' "${dir%/*}"
+    printf '%s\n' "${dir:-/}"
 }
 ```
 
diff --git a/test.sh b/test.sh
index 8be55c8..debf2da 100755
--- a/test.sh
+++ b/test.sh
@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-# shellcheck source=/dev/null
+# shellcheck source=/dev/null disable=2178,2128
 #
 # Tests for the Pure Bash Bible.
 
@@ -117,6 +117,18 @@ test_count() {
 test_dirname() {
     result="$(dirname "/home/black/Pictures/Wallpapers/1.jpg")"
     assert_equals "$result" "/home/black/Pictures/Wallpapers"
+
+    result="$(dirname "/")"
+    assert_equals "$result" "/"
+
+    result="$(dirname "/foo")"
+    assert_equals "$result" "/"
+
+    result="$(dirname ".")"
+    assert_equals "$result" "."
+
+    result="$(dirname "/foo/foo")"
+    assert_equals "$result" "/foo"
 }
 
 test_basename() {