diff --git a/src/requests/models.py b/src/requests/models.py
index c4b25fa079..4e38f86ae5 100644
--- a/src/requests/models.py
+++ b/src/requests/models.py
@@ -119,8 +119,15 @@ def _encode_params(data):
         elif hasattr(data, "__iter__"):
             result = []
             for k, vs in to_key_val_list(data):
-                if isinstance(vs, basestring) or not hasattr(vs, "__iter__"):
+                if isinstance(vs, (str, bytes)) or not hasattr(vs, '__iter__'):
                     vs = [vs]
+                
+                # --- START FIX ---
+                # If vs is an empty list [], treat it as [''] so it yields one empty key
+                if not vs:
+                    vs = ['']
+                # --- END FIX ---
+
                 for v in vs:
                     if v is not None:
                         result.append(
diff --git a/tests/test_requests.py b/tests/test_requests.py
index 75d2deff2e..71a589bb14 100644
--- a/tests/test_requests.py
+++ b/tests/test_requests.py
@@ -119,6 +119,21 @@ def test_basic_building(self):
         assert pr.url == req.url
         assert pr.body == "life=42"
 
+    def test_params_with_empty_list_are_included(self):
+        """
+        Regression test for #6557: Ensure empty lists in params 
+        result in the key being present in the URL (e.g. key=).
+        """
+        url = 'http://example.com'
+        params = {'key': []}
+        req = requests.Request('GET', url, params=params)
+        prep = req.prepare()
+        
+        # Check if the key is present with an empty value
+        assert 'key=' in prep.url
+        # Check that it didn't literally print the brackets
+        assert 'key=[]' not in prep.url
+         
     @pytest.mark.parametrize("method", ("GET", "HEAD"))
     def test_no_content_length(self, httpbin, method):
         req = requests.Request(method, httpbin(method.lower())).prepare()