diff --git a/Sources/swift-parsing-benchmark/JSON.swift b/Sources/swift-parsing-benchmark/JSON.swift
index 18d4921d67..8227de22cb 100644
--- a/Sources/swift-parsing-benchmark/JSON.swift
+++ b/Sources/swift-parsing-benchmark/JSON.swift
@@ -70,7 +70,8 @@ let jsonSuite = BenchmarkSuite(name: "JSON") { suite in
     struct JSONObject: ParserPrinter {
       var body: some ParserPrinter<Substring.UTF8View, [String: JSONValue.Output]> {
         "{".utf8
-        Many(into: [String: JSONValue.Output]()) { object, pair in
+        Many(into: [String: JSONValue.Output]()) {
+          (object: inout [String: JSONValue.Output], pair: (String, JSONValue.Output)) in
           let (name, value) = pair
           object[name] = value
         } decumulator: { object in