diff --git a/CHANGES.md b/CHANGES.md
index a8b556cb7e8..628f7eb28ce 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -41,6 +41,8 @@
 
 <!-- Changes to blackd -->
 
+- fix a bug where a single character would cause a 500 (#3558)
+
 ### Integrations
 
 <!-- For example, Docker, GitHub Actions, pre-commit, editors -->
diff --git a/src/blackd/__init__.py b/src/blackd/__init__.py
index ba4750b8298..e478c276a5f 100644
--- a/src/blackd/__init__.py
+++ b/src/blackd/__init__.py
@@ -146,7 +146,8 @@ async def handle(request: web.Request, executor: Executor) -> web.Response:
         )
 
         # Preserve CRLF line endings
-        if req_str[req_str.find("\n") - 1] == "\r":
+        nl = req_str.find("\n")
+        if nl > 0 and req_str[nl - 1] == "\r":
             formatted_str = formatted_str.replace("\n", "\r\n")
             # If, after swapping line endings, nothing changed, then say so
             if formatted_str == req_str:
diff --git a/tests/test_blackd.py b/tests/test_blackd.py
index 5b6461f7685..9e82b5bb344 100644
--- a/tests/test_blackd.py
+++ b/tests/test_blackd.py
@@ -240,3 +240,9 @@ async def test_normalizes_line_endings(self) -> None:
             response = await self.client.post("/", data=data)
             self.assertEqual(await response.text(), expected)
             self.assertEqual(response.status, 200)
+
+    @unittest_run_loop
+    async def test_single_character(self) -> None:
+        response = await self.client.post("/", data="1")
+        self.assertEqual(await response.text(), "1")
+        self.assertEqual(response.status, 200)