diff --git a/src/requests/models.py b/src/requests/models.py
index 2d043f59cf..14e87407f1 100644
--- a/src/requests/models.py
+++ b/src/requests/models.py
@@ -575,9 +575,13 @@ def prepare_content_length(self, body):
         """Prepare Content-Length header based on request method and body"""
         if body is not None:
             length = super_len(body)
-            if length:
-                # If length exists, set it. Otherwise, we fallback
-                # to Transfer-Encoding: chunked.
+            if length is not None and "Transfer-Encoding" not in self.headers:
+                # Set Content-Length for any known length, including 0.
+                # A length of 0 is valid (e.g. data={'foo': None} encodes
+                # to an empty body) and must be sent to avoid falling back
+                # to Transfer-Encoding: chunked, which can cause servers
+                # to misinterpret the terminating chunk as a new request.
+                # Skip if Transfer-Encoding is already set (e.g. for streams).
                 self.headers["Content-Length"] = builtin_str(length)
         elif (
             self.method not in ("GET", "HEAD")
diff --git a/tests/test_requests.py b/tests/test_requests.py
index 257d9d7ab1..12379a9282 100644
--- a/tests/test_requests.py
+++ b/tests/test_requests.py
@@ -2229,6 +2229,24 @@ def test_chunked_upload_does_not_set_content_length_header(self, httpbin):
         assert "Transfer-Encoding" in prepared_request.headers
         assert "Content-Length" not in prepared_request.headers
 
+    def test_empty_body_from_none_values_sets_content_length_zero(self, httpbin):
+        """Ensure that a request body consisting only of None values sets
+        Content-Length: 0 instead of falling back to Transfer-Encoding: chunked.
+
+        When data={'foo': None}, the body encodes to an empty string.
+        Without Content-Length: 0, the adapter falls back to chunked encoding
+        and sends a terminating chunk ('0\\r\\n\\r\\n') that servers may
+        misinterpret as a second, malformed request.
+
+        See: https://github.com/psf/requests/issues/6122
+        """
+        url = httpbin("post")
+        r = requests.Request("POST", url, data={"foo": None})
+        prepared_request = r.prepare()
+        assert prepared_request.body == ""
+        assert prepared_request.headers["Content-Length"] == "0"
+        assert "Transfer-Encoding" not in prepared_request.headers
+
     def test_custom_redirect_mixin(self, httpbin):
         """Tests a custom mixin to overwrite ``get_redirect_target``.