diff --git a/docs/source/release-notes/unreleased.rst b/docs/source/release-notes/unreleased.rst
index 09b03b4..a094250 100644
--- a/docs/source/release-notes/unreleased.rst
+++ b/docs/source/release-notes/unreleased.rst
@@ -1,6 +1,7 @@
 2.x.y - 202z-aa-bb
 ------------------
 
-- *Add Items here*
+- Fix ``ParseResultBytes.unsplit()`` raising ``AttributeError`` for
+  references without an authority, such as ``/a`` or ``mailto:x``.
 
 .. links below here
diff --git a/src/rfc3986/parseresult.py b/src/rfc3986/parseresult.py
index 551d1c6..168045f 100644
--- a/src/rfc3986/parseresult.py
+++ b/src/rfc3986/parseresult.py
@@ -404,9 +404,12 @@ def from_string(
         )
 
     @property
-    def authority(self) -> bytes:
+    def authority(self) -> t.Optional[bytes]:
         """Return the normalized authority."""
-        return self.reference.authority.encode(self.encoding)
+        authority = self.reference.authority
+        if authority is None:
+            return None
+        return authority.encode(self.encoding)
 
     def copy_with(
         self,
diff --git a/tests/test_parseresult.py b/tests/test_parseresult.py
index 5788448..3217898 100644
--- a/tests/test_parseresult.py
+++ b/tests/test_parseresult.py
@@ -166,6 +166,18 @@ def test_unsplit(self):
         idna_encoded = SNOWMAN_IDNA_HOST.encode("utf-8") + b"/path"
         assert uri.unsplit(use_idna=True) == idna_encoded
 
+    @pytest.mark.parametrize(
+        "reference", [b"/a", b"a", b"mailto:x", b"?q", b"#f"]
+    )
+    def test_unsplit_reference_without_authority(self, reference):
+        uri = pr.ParseResultBytes.from_string(reference)
+        assert uri.authority is None
+        assert uri.unsplit() == reference
+        assert (
+            pr.ParseResult.from_string(reference.decode()).unsplit()
+            == reference.decode()
+        )
+
     def test_eager_normalization_from_string(self):
         uri = pr.ParseResultBytes.from_string(
             b"http://" + SNOWMAN + b".com/path",