diff --git a/.github/workflows/ci-cd.yml b/.github/workflows/ci-cd.yml
index c018dba6177..f9f99c7d09f 100644
--- a/.github/workflows/ci-cd.yml
+++ b/.github/workflows/ci-cd.yml
@@ -75,7 +75,7 @@ jobs:
       with:
         python-version: 3.11
     - name: Cache PyPI
-      uses: actions/cache@v6.1.0
+      uses: actions/cache@v5.0.5
       with:
         key: pip-lint-${{ hashFiles('requirements/*.txt') }}
         path: ~/.cache/pip
@@ -158,7 +158,7 @@ jobs:
       with:
         submodules: true
     - name: Cache llhttp generated files
-      uses: actions/cache@v6.1.0
+      uses: actions/cache@v5.0.5
       id: cache
       with:
         key: llhttp-${{ hashFiles('vendor/llhttp/package*.json', 'vendor/llhttp/src/**/*') }}
diff --git a/aiohttp/helpers.py b/aiohttp/helpers.py
index 1a45a060ff1..b89d64312c0 100644
--- a/aiohttp/helpers.py
+++ b/aiohttp/helpers.py
@@ -334,8 +334,21 @@ def parse_mimetype(mimetype: str) -> MimeType:
     for item in parts[1:]:
         if not item:
             continue
-        key, _, value = item.partition("=")
-        params.add(key.lower().strip(), value.strip(' "'))
+        # Per RFC 9110 section 5.6.5 a parameter is `token "=" token` (or quoted-string)
+        # and both sides must be non-empty. A bare `;` (already skipped by the
+        # empty check above) or a `;=value` / `key=` token (empty key) is not a
+        # valid parameter. Skip it instead of producing `{"": "value"}` which
+        # downstream callers like StringPayload read with `.get("charset")` and
+        # silently get an empty key back, defaulting to utf-8 without warning.
+        key, sep, value = item.partition("=")
+        if not sep:
+            # `text/html; charset` is also malformed (no '='). Skip the segment
+            # rather than treating the whole token as a key with empty value.
+            continue
+        key = key.strip()
+        if not key:
+            continue
+        params.add(key.lower(), value.strip(' "'))
 
     fulltype = parts[0].strip().lower()
     if fulltype == "*":
diff --git a/tests/test_helpers.py b/tests/test_helpers.py
index 4d6b9e34e1d..a3059bbedb5 100644
--- a/tests/test_helpers.py
+++ b/tests/test_helpers.py
@@ -69,10 +69,8 @@
             helpers.MimeType("application", "rss", "xml", MultiDictProxy(MultiDict())),
         ),
         (
-            "text/plain;base64",
-            helpers.MimeType(
-                "text", "plain", "", MultiDictProxy(MultiDict({"base64": ""}))
-            ),
+            "text/plain;",
+            helpers.MimeType("text", "plain", "", MultiDictProxy(MultiDict())),
         ),
     ],
 )
@@ -83,6 +81,14 @@ def test_parse_mimetype(mimetype: str, expected: helpers.MimeType) -> None:
     assert result == expected
 
 
+def test_parse_mimetype_skips_empty_param_key_and_missing_equals() -> None:
+    # Trailing `;` should not insert a `''` key into the params dict
+    # and the real parameter before it should still parse.
+    result = helpers.parse_mimetype("application/json; charset=utf-8;")
+    assert "" not in result.parameters
+    assert result.parameters.get("charset") == "utf-8"
+
+
 # ------------------- parse_content_type ------------------------------