diff --git a/src/eligibility_signposting_api/app.py b/src/eligibility_signposting_api/app.py
index 9073bb779..26be07c99 100644
--- a/src/eligibility_signposting_api/app.py
+++ b/src/eligibility_signposting_api/app.py
@@ -29,6 +29,7 @@ def lambda_handler(event: LambdaEvent, context: LambdaContext) -> dict[str, Any]
     app = create_app()
     app.debug = config()["log_level"] == logging.DEBUG
     handler = Mangum(WsgiToAsgi(app), lifespan="off")
+    handler.config["text_mime_types"].append("application/fhir+json")
     return handler(event, context)
 
 
diff --git a/tests/integration/lambda/test_app_running_as_lambda.py b/tests/integration/lambda/test_app_running_as_lambda.py
index 0e9308796..b157fe44f 100644
--- a/tests/integration/lambda/test_app_running_as_lambda.py
+++ b/tests/integration/lambda/test_app_running_as_lambda.py
@@ -124,37 +124,34 @@ def test_install_and_call_flask_lambda_with_unknown_nhs_number(
         timeout=10,
     )
 
-    decoded_body_bytes = base64.b64decode(response.text)
-    decoded_body_json = json.loads(decoded_body_bytes.decode("utf-8"))
-
     assert_that(
         response,
         is_response()
         .with_status_code(HTTPStatus.NOT_FOUND)
-        .with_headers(has_entries({"Content-Type": "application/fhir+json"})),
-    )
-
-    # Then
-    assert_that(
-        decoded_body_json,
-        has_entries(
-            resourceType=equal_to("OperationOutcome"),
-            issue=contains_exactly(
+        .with_headers(has_entries({"Content-Type": "application/fhir+json"}))
+        .and_body(
+            is_json_that(
                 has_entries(
-                    severity="error",
-                    code="processing",
-                    diagnostics=f"NHS Number '{nhs_number!s}' was not recognised by the Eligibility Signposting API",
-                    details={
-                        "coding": [
-                            {
-                                "system": "https://fhir.nhs.uk/STU3/ValueSet/Spine-ErrorOrWarningCode-1",
-                                "code": "REFERENCE_NOT_FOUND",
-                                "display": "The given NHS number was not found in our datasets. "
-                                "This could be because the number is incorrect or "
-                                "some other reason we cannot process that number.",
-                            }
-                        ]
-                    },
+                    resourceType="OperationOutcome",
+                    issue=contains_exactly(
+                        has_entries(
+                            severity="error",
+                            code="processing",
+                            diagnostics=f"NHS Number '{nhs_number!s}' was not "
+                            f"recognised by the Eligibility Signposting API",
+                            details={
+                                "coding": [
+                                    {
+                                        "system": "https://fhir.nhs.uk/STU3/ValueSet/Spine-ErrorOrWarningCode-1",
+                                        "code": "REFERENCE_NOT_FOUND",
+                                        "display": "The given NHS number was not found in our datasets. "
+                                        "This could be because the number is incorrect or "
+                                        "some other reason we cannot process that number.",
+                                    }
+                                ]
+                            },
+                        )
+                    ),
                 )
             ),
         ),