diff --git a/walk/filesystem.go b/walk/filesystem.go
index 18cc3dfc..c14fc451 100644
--- a/walk/filesystem.go
+++ b/walk/filesystem.go
@@ -20,11 +20,11 @@ func (f filesystemWalker) Walk(_ context.Context, fn WalkFunc) error {
 	relPathOffset := len(f.root) + 1
 
 	relPathFn := func(path string) (string, error) {
-		// quick optimisation for the majority of use cases
-		// todo check that root is a prefix in path?
-		if len(path) >= relPathOffset {
+		// quick optimization for the majority of use cases
+		if len(path) >= relPathOffset && path[:len(f.root)] == f.root {
 			return path[relPathOffset:], nil
 		}
+		// fallback to proper relative path resolution
 		return filepath.Rel(f.root, path)
 	}
 
diff --git a/walk/filesystem_test.go b/walk/filesystem_test.go
new file mode 100644
index 00000000..202f0595
--- /dev/null
+++ b/walk/filesystem_test.go
@@ -0,0 +1,95 @@
+package walk
+
+import (
+	"context"
+	"os"
+	"testing"
+
+	"git.numtide.com/numtide/treefmt/test"
+	"github.com/stretchr/testify/require"
+)
+
+var examplesPaths = []string{
+	".",
+	"elm",
+	"elm/elm.json",
+	"elm/src",
+	"elm/src/Main.elm",
+	"go",
+	"go/go.mod",
+	"go/main.go",
+	"haskell",
+	"haskell/CHANGELOG.md",
+	"haskell/Foo.hs",
+	"haskell/Main.hs",
+	"haskell/Nested",
+	"haskell/Nested/Foo.hs",
+	"haskell/Setup.hs",
+	"haskell/haskell.cabal",
+	"haskell/treefmt.toml",
+	"haskell-frontend",
+	"haskell-frontend/CHANGELOG.md",
+	"haskell-frontend/Main.hs",
+	"haskell-frontend/Setup.hs",
+	"haskell-frontend/haskell-frontend.cabal",
+	"html",
+	"html/index.html",
+	"html/scripts",
+	"html/scripts/.gitkeep",
+	"javascript",
+	"javascript/source",
+	"javascript/source/hello.js",
+	"nix",
+	"nix/sources.nix",
+	"nixpkgs.toml",
+	"python",
+	"python/main.py",
+	"python/requirements.txt",
+	"python/virtualenv_proxy.py",
+	"ruby",
+	"ruby/bundler.rb",
+	"rust",
+	"rust/Cargo.toml",
+	"rust/src",
+	"rust/src/main.rs",
+	"shell",
+	"shell/foo.sh",
+	"terraform",
+	"terraform/main.tf",
+	"terraform/two.tf",
+	"touch.toml",
+	"treefmt.toml",
+	"yaml",
+	"yaml/test.yaml",
+}
+
+func TestFilesystemWalker_Walk(t *testing.T) {
+	tempDir := test.TempExamples(t)
+
+	paths := make(chan string, 1)
+	go func() {
+		paths <- tempDir
+		close(paths)
+	}()
+
+	as := require.New(t)
+
+	walker, err := NewFilesystem(tempDir, paths)
+	as.NoError(err)
+
+	idx := 0
+	err = walker.Walk(context.Background(), func(file *File, err error) error {
+		as.Equal(examplesPaths[idx], file.RelPath)
+		idx += 1
+		return nil
+	})
+	as.NoError(err)
+
+	// capture current cwd, so we can replace it after the test is finished
+	cwd, err := os.Getwd()
+	as.NoError(err)
+	t.Cleanup(func() {
+		// return to the previous working directory
+		as.NoError(os.Chdir(cwd))
+	})
+}