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/CHANGES/12994.bugfix.rst b/CHANGES/12994.bugfix.rst
new file mode 100644
index 00000000000..6221cf43a3a
--- /dev/null
+++ b/CHANGES/12994.bugfix.rst
@@ -0,0 +1 @@
+Fixed :py:class:`~aiohttp.http_parser.DeflateBuffer` to treat an empty ``feed_data`` call as a
diff --git a/CONTRIBUTORS.txt b/CONTRIBUTORS.txt
index 65c20d023e7..719f3eac01e 100644
--- a/CONTRIBUTORS.txt
+++ b/CONTRIBUTORS.txt
@@ -165,6 +165,7 @@ Hans Adema
 Harmon Y.
 Harry Liu
 Hiroshi Ogawa
+Hrach Shahumyan
 Hrishikesh Paranjape
 Hu Bo
 Hugh Young
diff --git a/aiohttp/http_parser.py b/aiohttp/http_parser.py
index 6abbe04feae..9a8d3538e88 100644
--- a/aiohttp/http_parser.py
+++ b/aiohttp/http_parser.py
@@ -1137,7 +1137,7 @@ def feed_data(self, chunk: bytes) -> bool:
         # RFC1950
         # bits 0..3 = CM = 0b1000 = 8 = "deflate"
         # bits 4..7 = CINFO = 1..7 = windows size.
-        if (
+        if chunk and (
             not self._started_decoding
             and self.encoding == "deflate"
             and chunk[0] & 0xF != 8
diff --git a/tests/test_http_parser.py b/tests/test_http_parser.py
index e61af88cc16..7cd66c3e19b 100644
--- a/tests/test_http_parser.py
+++ b/tests/test_http_parser.py
@@ -857,15 +857,15 @@ def test_request_chunked(parser: HttpRequestParser) -> None:
 
 
 def test_te_header_non_ascii(parser: HttpRequestParser) -> None:
-    # K = Kelvin sign, not valid ascii.
-    text = "GET /test HTTP/1.1\r\nHost: a\r\nTransfer-Encoding: chunKed\r\n\r\n"
+    # K = Kelvin sign, not valid ascii.
+    text = "GET /test HTTP/1.1\r\nHost: a\r\nTransfer-Encoding: chunKed\r\n\r\n"
     with pytest.raises(http_exceptions.BadHttpMessage):
         parser.feed_data(text.encode())
 
 
 def test_upgrade_header_non_ascii(parser: HttpRequestParser) -> None:
-    # K = Kelvin sign, not valid ascii.
-    text = "GET /test HTTP/1.1\r\nHost: a\r\nUpgrade: websocKet\r\n\r\n"
+    # K = Kelvin sign, not valid ascii.
+    text = "GET /test HTTP/1.1\r\nHost: a\r\nUpgrade: websocKet\r\n\r\n"
     messages, upgrade, tail = parser.feed_data(text.encode())
     assert not upgrade
 
@@ -2982,6 +2982,27 @@ async def test_feed_data(self, protocol: BaseProtocol) -> None:
         dbuf.feed_data(b"xxxx")
         assert [b"line"] == list(buf._buffer)
 
+    async def test_feed_data_empty(self, protocol: BaseProtocol) -> None:
+        """feed_data(b"") must not raise even on a fresh deflate stream.
+
+        The chunked-transfer decoder calls feed_data(b"") to give a paused
+        decoder another chance to make progress. The CM-byte sniff in
+        feed_data would previously read chunk[0] on an empty chunk and raise
+        IndexError. Regression test for #12994.
+        """
+        buf = aiohttp.StreamReader(
+            protocol, DEFAULT_CHUNK_SIZE, loop=asyncio.get_running_loop()
+        )
+        dbuf = DeflateBuffer(buf, "deflate")
+
+        # Should be a no-op, returning False (no more data).
+        assert dbuf.feed_data(b"") is False
+        # No bytes pushed to the downstream stream.
+        assert list(buf._buffer) == []
+        # Decoder was not switched to suppress_deflate_header by the empty
+        # chunk.
+        assert dbuf._started_decoding is False
+
     async def test_feed_data_err(self, protocol: BaseProtocol) -> None:
         buf = aiohttp.StreamReader(protocol, 2**16, loop=asyncio.get_running_loop())
         dbuf = DeflateBuffer(buf, "deflate")