diff --git a/starlette_exporter/labels.py b/starlette_exporter/labels.py
index 1701638..4bdcda3 100644
--- a/starlette_exporter/labels.py
+++ b/starlette_exporter/labels.py
@@ -2,7 +2,6 @@
 from typing import Any, Callable, Iterable, Optional, Dict
 
 from starlette.requests import Request
-from starlette.types import Message
 
 
 class ResponseHeaderLabel:
@@ -13,7 +12,7 @@ class ResponseHeaderLabel:
     def __init__(
         self, key: str, allowed_values: Optional[Iterable] = None, default: str = ""
     ) -> None:
-        self.key = key
+        self.key = key.lower()
         self.default = default
         self.allowed_values = allowed_values
 
diff --git a/starlette_exporter/middleware.py b/starlette_exporter/middleware.py
index 5a6ad01..5a68761 100644
--- a/starlette_exporter/middleware.py
+++ b/starlette_exporter/middleware.py
@@ -300,7 +300,7 @@ def _response_label_values(self, message: Message) -> List[str]:
 
         # create a dict of headers to make it easy to find keys
         headers = {
-            k.decode("utf-8"): v.decode("utf-8")
+            k.decode("utf-8").lower(): v.decode("utf-8")
             for (k, v) in message.get("headers", ())
         }
 
@@ -405,6 +405,10 @@ async def wrapped_send(message: Message) -> None:
             await self.app(scope, receive, wrapped_send)
         except Exception as e:
             status_code = 500
+
+            # during an unhandled exception, populate response labels with empty strings.
+            response_labels = self._response_label_values({})
+
             exception = e
         finally:
             # Decrement 'requests_in_progress' gauge after response sent
diff --git a/tests/test_middleware.py b/tests/test_middleware.py
index bc25401..8364c6e 100644
--- a/tests/test_middleware.py
+++ b/tests/test_middleware.py
@@ -67,7 +67,7 @@ def httpstatus_response(request):
         )
 
         async def error(request):
-            raise HTTPException(status_code=500, detail="this is a test error")
+            raise HTTPException(status_code=500, detail="this is a test error", headers={"foo":"baz"})
 
         app.add_route("/500", error)
         app.add_route("/500/{test_param}", error)
@@ -804,6 +804,46 @@ def test_from_response_header(self, testapp):
             in metrics
         ), metrics
 
+    def test_from_response_header_case_insensitive(self, testapp):
+        """test with the library-provided from_response_header function with a capitalized header key."""
+        labels = {"foo": from_response_header("Foo"), "hello": "world"}
+        client = TestClient(testapp(labels=labels))
+        client.get("/200")
+        metrics = client.get("/metrics").content.decode()
+
+        assert (
+            """starlette_requests_total{app_name="starlette",foo="baz",hello="world",method="GET",path="/200",status_code="200"} 1.0"""
+            in metrics
+        ), metrics
+
+    def test_from_response_header_http_exception(self, testapp):
+        """test from_response_header against an endpoint that raises an HTTPException"""
+        labels = {"foo": from_response_header("foo"), "hello": "world"}
+        client = TestClient(testapp(labels=labels))
+        client.get("/500")
+        metrics = client.get("/metrics").content.decode()
+        
+        assert (
+            """starlette_requests_total{app_name="starlette",foo="baz",hello="world",method="GET",path="/500",status_code="500"} 1.0"""
+            in metrics
+        ), metrics
+    
+    def test_from_response_header_unhandled_exception(self, testapp):
+        """test from_response_header function against an endpoint that raises an unhandled exception"""
+        labels = {"foo": from_response_header("foo"), "hello": "world"}
+        client = TestClient(testapp(labels=labels))
+
+        # make the test call. This raises an error but will still populate metrics.
+        with pytest.raises(KeyError, match="value_error"):
+            client.get("/unhandled")
+
+        metrics = client.get("/metrics").content.decode()
+
+        assert (
+            """starlette_requests_total{app_name="starlette",foo="",hello="world",method="GET",path="/unhandled",status_code="500"} 1.0"""
+            in metrics
+        ), metrics
+
     def test_from_response_header_default(self, testapp):
         """test with the library-provided from_response_header function, with a missing header
         (testing the default value)"""