diff --git a/mountpoint-s3/tests/reftests/generators.rs b/mountpoint-s3/tests/reftests/generators.rs
index 763bdb476..13e053689 100644
--- a/mountpoint-s3/tests/reftests/generators.rs
+++ b/mountpoint-s3/tests/reftests/generators.rs
@@ -90,9 +90,16 @@ impl FileContent {
     }
 }
 
+/// Represents a file system tree.
+///
+/// The root should always be the [TreeNode::Directory] variant.
 #[derive(Clone)]
 pub enum TreeNode {
+    /// File node in a tree.
+    ///
+    /// A file node must never be at the root of the tree, and is given a name by its parent (a [TreeNode::Directory]).
     File(FileContent),
+    /// Directory node in the tree.
     Directory(BTreeMap<Name, TreeNode>),
 }
 
@@ -115,19 +122,26 @@ impl Debug for TreeNode {
     }
 }
 
+/// Generates a tree of directories and files.
+///
+/// Leaves are always [TreeNode::File] or an empty [TreeNode::Directory].
+/// Parents are always [TreeNode::Directory].
 pub fn gen_tree(depth: u32, max_size: u32, max_items: u32, max_width: usize) -> impl Strategy<Value = TreeNode> {
-    let leaf = prop_oneof![any::<FileContent>().prop_map(TreeNode::File),];
-    leaf.prop_recursive(
+    let leaf = any::<FileContent>().prop_map(TreeNode::File);
+
+    let strategy = leaf.prop_recursive(
         depth,     // levels
         max_size,  // max number of nodes
         max_items, // number of items per collection
         move |inner| {
-            prop_oneof![
-                // Take the inner strategy and make the recursive cases.
-                prop::collection::btree_map(any::<Name>(), inner, 0..max_width).prop_map(TreeNode::Directory),
-            ]
+            // Take the inner strategy and make the recursive cases.
+            // Since the size of the tree could be zero, this also introduces directories as leaves.
+            prop::collection::btree_map(any::<Name>(), inner, 0..max_width).prop_map(TreeNode::Directory)
         },
-    )
+    );
+
+    // Ensure the root is always a directory by wrapping the inner strategy
+    prop::collection::btree_map(any::<Name>(), strategy, 0..max_width).prop_map(TreeNode::Directory)
 }
 
 /// Take a generated tree and create the corresponding S3 namespace (list of keys)
@@ -135,10 +149,8 @@ pub fn flatten_tree(node: TreeNode) -> Vec<(String, MockObject)> {
     fn aux(node: TreeNode, path: String, acc: &mut Vec<(String, MockObject)>) {
         match node {
             TreeNode::File(content) => {
-                // Don't create an empty key if a [TreeNode::File] is the root of the tree
-                if !path.is_empty() {
-                    acc.push((path, content.to_mock_object()));
-                }
+                assert!(!path.is_empty(), "file node should never be created at root");
+                acc.push((path, content.to_mock_object()));
             }
             TreeNode::Directory(contents) => {
                 for (name, child) in contents {
diff --git a/mountpoint-s3/tests/reftests/harness.rs b/mountpoint-s3/tests/reftests/harness.rs
index 045362024..a499ecd3e 100644
--- a/mountpoint-s3/tests/reftests/harness.rs
+++ b/mountpoint-s3/tests/reftests/harness.rs
@@ -947,7 +947,7 @@ mod mutations {
     #[test]
     fn regression_overwrite() {
         run_test(
-            TreeNode::File(FileContent(0, FileSize::Small(0))),
+            TreeNode::Directory(BTreeMap::from([])),
             vec![
                 Op::WriteFile("-a".into(), DirectoryIndex(0), FileContent(0, FileSize::Small(0))),
                 Op::WriteFile("-a".into(), DirectoryIndex(0), FileContent(0, FileSize::Small(0))),
@@ -959,7 +959,7 @@ mod mutations {
     #[test]
     fn regression_empty_file() {
         run_test(
-            TreeNode::File(FileContent(0, FileSize::Small(0))),
+            TreeNode::Directory(BTreeMap::from([])),
             vec![
                 Op::CreateFile("a".into(), DirectoryIndex(0), FileContent(0, FileSize::Small(0))),
                 Op::FinishWrite(InflightWriteIndex(0)),
@@ -988,7 +988,7 @@ mod mutations {
     #[test]
     fn regression_unlink_local_directory() {
         run_test(
-            TreeNode::File(FileContent(0, FileSize::Small(0))),
+            TreeNode::Directory(BTreeMap::from([])),
             vec![
                 Op::CreateDirectory(DirectoryIndex(0), "a".into()),
                 Op::UnlinkFile(DirectoryIndex(0), ChildIndex(0)),
@@ -1161,7 +1161,7 @@ mod mutations {
     #[test]
     fn regression_mkdir_put() {
         run_test(
-            TreeNode::File(FileContent(0, FileSize::Small(0))),
+            TreeNode::Directory(BTreeMap::from([])),
             vec![
                 Op::CreateDirectory(DirectoryIndex(0), "a".into()),
                 Op::PutObject(DirectoryIndex(0), "a".into(), FileContent(0, FileSize::Small(0))),
@@ -1173,7 +1173,7 @@ mod mutations {
     #[test]
     fn regression_put_over_open_file() {
         run_test(
-            TreeNode::File(FileContent(0, FileSize::Small(0))),
+            TreeNode::Directory(BTreeMap::from([])),
             vec![
                 Op::CreateFile("a".into(), DirectoryIndex(0), FileContent(0, FileSize::Small(0))),
                 Op::PutObject(DirectoryIndex(0), "a".into(), FileContent(0, FileSize::Small(0))),
@@ -1185,7 +1185,7 @@ mod mutations {
     #[test]
     fn regression_put_over_open_directory() {
         run_test(
-            TreeNode::File(FileContent(0, FileSize::Small(0))),
+            TreeNode::Directory(BTreeMap::from([])),
             vec![
                 Op::CreateDirectory(DirectoryIndex(0), "a".into()),
                 Op::PutObject(DirectoryIndex(0), "a".into(), FileContent(0, FileSize::Small(0))),
@@ -1236,7 +1236,7 @@ mod mutations {
     #[test]
     fn regression_put_into_directory1() {
         run_test(
-            TreeNode::File(FileContent(0, FileSize::Small(0))),
+            TreeNode::Directory(BTreeMap::from([])),
             vec![
                 Op::CreateDirectory(DirectoryIndex(0), "a".into()),
                 Op::CreateDirectory(DirectoryIndex(0), "-".into()),
@@ -1298,7 +1298,7 @@ mod mutations {
     #[test]
     fn regression_unlink_newly_put_object() {
         run_test(
-            TreeNode::File(FileContent(0, FileSize::Small(0))),
+            TreeNode::Directory(BTreeMap::from([])),
             vec![
                 Op::CreateDirectory(DirectoryIndex(0), "a".into()),
                 Op::PutObject(DirectoryIndex(1), "a".into(), FileContent(0, FileSize::Small(0))),