diff --git a/mfer/manifest.go b/mfer/manifest.go
index 357ab08..130b611 100644
--- a/mfer/manifest.go
+++ b/mfer/manifest.go
@@ -74,11 +74,17 @@ func (m *manifest) addInputPath(inputPath string) error {
 }
 
 func (m *manifest) addInputFS(f afero.Fs) error {
+	if f == nil {
+		return errors.New("filesystem cannot be nil")
+	}
+	// Validate filesystem is accessible by statting root
+	if _, err := f.Stat("/"); err != nil {
+		return fmt.Errorf("filesystem not accessible: %w", err)
+	}
 	if m.sourceFS == nil {
 		m.sourceFS = make([]afero.Fs, 0)
 	}
 	m.sourceFS = append(m.sourceFS, f)
-	// FIXME do some sort of check on f here?
 	return nil
 }