diff --git a/lib/std/fs/path.zig b/lib/std/fs/path.zig
index 5e418e8e5c..1bf2211372 100644
--- a/lib/std/fs/path.zig
+++ b/lib/std/fs/path.zig
@@ -749,8 +749,12 @@ fn testResolvePosix(paths: []const []const u8, expected: []const u8) !void {
     return testing.expect(mem.eql(u8, actual, expected));
 }
 
+/// Strip the last component from a file path.
+///
 /// If the path is a file in the current directory (no directory component)
-/// then returns null
+/// then returns null.
+///
+/// If the path is the root directory, returns null.
 pub fn dirname(path: []const u8) ?[]const u8 {
     if (builtin.os.tag == .windows) {
         return dirnameWindows(path);
@@ -765,19 +769,19 @@ pub fn dirnameWindows(path: []const u8) ?[]const u8 {
 
     const root_slice = diskDesignatorWindows(path);
     if (path.len == root_slice.len)
-        return path;
+        return null;
 
     const have_root_slash = path.len > root_slice.len and (path[root_slice.len] == '/' or path[root_slice.len] == '\\');
 
     var end_index: usize = path.len - 1;
 
-    while ((path[end_index] == '/' or path[end_index] == '\\') and end_index > root_slice.len) {
+    while (path[end_index] == '/' or path[end_index] == '\\') {
         if (end_index == 0)
             return null;
         end_index -= 1;
     }
 
-    while (path[end_index] != '/' and path[end_index] != '\\' and end_index > root_slice.len) {
+    while (path[end_index] != '/' and path[end_index] != '\\') {
         if (end_index == 0)
             return null;
         end_index -= 1;
@@ -800,7 +804,7 @@ pub fn dirnamePosix(path: []const u8) ?[]const u8 {
     var end_index: usize = path.len - 1;
     while (path[end_index] == '/') {
         if (end_index == 0)
-            return path[0..1];
+            return null;
         end_index -= 1;
     }
 
@@ -810,7 +814,7 @@ pub fn dirnamePosix(path: []const u8) ?[]const u8 {
         end_index -= 1;
     }
 
-    if (end_index == 0 and path[end_index] == '/')
+    if (end_index == 0 and path[0] == '/')
         return path[0..1];
 
     if (end_index == 0)
@@ -823,8 +827,10 @@ test "dirnamePosix" {
     testDirnamePosix("/a/b/c", "/a/b");
     testDirnamePosix("/a/b/c///", "/a/b");
     testDirnamePosix("/a", "/");
-    testDirnamePosix("/", "/");
-    testDirnamePosix("////", "/");
+    testDirnamePosix("/", null);
+    testDirnamePosix("//", null);
+    testDirnamePosix("///", null);
+    testDirnamePosix("////", null);
     testDirnamePosix("", null);
     testDirnamePosix("a", null);
     testDirnamePosix("a/", null);
@@ -832,27 +838,27 @@ test "dirnamePosix" {
 }
 
 test "dirnameWindows" {
-    testDirnameWindows("c:\\", "c:\\");
+    testDirnameWindows("c:\\", null);
     testDirnameWindows("c:\\foo", "c:\\");
     testDirnameWindows("c:\\foo\\", "c:\\");
     testDirnameWindows("c:\\foo\\bar", "c:\\foo");
     testDirnameWindows("c:\\foo\\bar\\", "c:\\foo");
     testDirnameWindows("c:\\foo\\bar\\baz", "c:\\foo\\bar");
-    testDirnameWindows("\\", "\\");
+    testDirnameWindows("\\", null);
     testDirnameWindows("\\foo", "\\");
     testDirnameWindows("\\foo\\", "\\");
     testDirnameWindows("\\foo\\bar", "\\foo");
     testDirnameWindows("\\foo\\bar\\", "\\foo");
     testDirnameWindows("\\foo\\bar\\baz", "\\foo\\bar");
-    testDirnameWindows("c:", "c:");
-    testDirnameWindows("c:foo", "c:");
-    testDirnameWindows("c:foo\\", "c:");
+    testDirnameWindows("c:", null);
+    testDirnameWindows("c:foo", null);
+    testDirnameWindows("c:foo\\", null);
     testDirnameWindows("c:foo\\bar", "c:foo");
     testDirnameWindows("c:foo\\bar\\", "c:foo");
     testDirnameWindows("c:foo\\bar\\baz", "c:foo\\bar");
     testDirnameWindows("file:stream", null);
     testDirnameWindows("dir\\file:stream", "dir");
-    testDirnameWindows("\\\\unc\\share", "\\\\unc\\share");
+    testDirnameWindows("\\\\unc\\share", null);
     testDirnameWindows("\\\\unc\\share\\foo", "\\\\unc\\share\\");
     testDirnameWindows("\\\\unc\\share\\foo\\", "\\\\unc\\share\\");
     testDirnameWindows("\\\\unc\\share\\foo\\bar", "\\\\unc\\share\\foo");
@@ -862,8 +868,8 @@ test "dirnameWindows" {
     testDirnameWindows("/a/b", "/a");
     testDirnameWindows("/a", "/");
     testDirnameWindows("", null);
-    testDirnameWindows("/", "/");
-    testDirnameWindows("////", "/");
+    testDirnameWindows("/", null);
+    testDirnameWindows("////", null);
     testDirnameWindows("foo", null);
 }