diff --git a/src/libfetchers/fetchers.cc b/src/libfetchers/fetchers.cc
index 483796f0b..a06d931db 100644
--- a/src/libfetchers/fetchers.cc
+++ b/src/libfetchers/fetchers.cc
@@ -3,6 +3,7 @@
 #include "input-accessor.hh"
 #include "source-path.hh"
 #include "fetch-to-store.hh"
+#include "json-utils.hh"
 
 #include <nlohmann/json.hpp>
 
@@ -412,3 +413,20 @@ std::string publicKeys_to_string(const std::vector<PublicKey>& publicKeys)
 }
 
 }
+
+namespace nlohmann {
+
+using namespace nix;
+
+fetchers::PublicKey adl_serializer<fetchers::PublicKey>::from_json(const json & json) {
+    auto type = optionalValueAt(json, "type").value_or("ssh-ed25519");
+    auto key = valueAt(json, "key");
+    return fetchers::PublicKey { getString(type), getString(key) };
+}
+
+void adl_serializer<fetchers::PublicKey>::to_json(json & json, fetchers::PublicKey p) {
+    json["type"] = p.type;
+    json["key"] = p.key;
+}
+
+}
diff --git a/src/libfetchers/fetchers.hh b/src/libfetchers/fetchers.hh
index cd11f9eae..bb21c68cc 100644
--- a/src/libfetchers/fetchers.hh
+++ b/src/libfetchers/fetchers.hh
@@ -4,6 +4,7 @@
 #include "types.hh"
 #include "hash.hh"
 #include "canon-path.hh"
+#include "json-impls.hh"
 #include "attrs.hh"
 #include "url.hh"
 
@@ -230,8 +231,9 @@ struct PublicKey
     std::string type = "ssh-ed25519";
     std::string key;
 };
-NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT(PublicKey, type, key)
 
 std::string publicKeys_to_string(const std::vector<PublicKey>&);
 
 }
+
+JSON_IMPL(fetchers::PublicKey)
diff --git a/src/libutil/json-utils.cc b/src/libutil/json-utils.cc
index 7a7264a9a..1b911bf75 100644
--- a/src/libutil/json-utils.cc
+++ b/src/libutil/json-utils.cc
@@ -30,14 +30,12 @@ const nlohmann::json & valueAt(
     return map.at(key);
 }
 
-std::optional<nlohmann::json> optionalValueAt(const nlohmann::json & value, const std::string & key)
+std::optional<nlohmann::json> optionalValueAt(const nlohmann::json::object_t & map, const std::string & key)
 {
-    try {
-        auto & v = valueAt(value, key);
-        return v.get<nlohmann::json>();
-    } catch (...) {
+    if (!map.contains(key))
         return std::nullopt;
-    }
+
+    return std::optional { map.at(key) };
 }
 
 
diff --git a/src/libutil/json-utils.hh b/src/libutil/json-utils.hh
index 2024624f4..08c98cc8c 100644
--- a/src/libutil/json-utils.hh
+++ b/src/libutil/json-utils.hh
@@ -23,7 +23,7 @@ const nlohmann::json & valueAt(
     const nlohmann::json::object_t & map,
     const std::string & key);
 
-std::optional<nlohmann::json> optionalValueAt(const nlohmann::json & value, const std::string & key);
+std::optional<nlohmann::json> optionalValueAt(const nlohmann::json::object_t & value, const std::string & key);
 
 /**
  * Downcast the json object, failing with a nice error if the conversion fails.
diff --git a/tests/unit/libutil/json-utils.cc b/tests/unit/libutil/json-utils.cc
index ffa667806..ec653fff5 100644
--- a/tests/unit/libutil/json-utils.cc
+++ b/tests/unit/libutil/json-utils.cc
@@ -160,4 +160,16 @@ TEST(getBoolean, wrongAssertions) {
     ASSERT_THROW(getBoolean(valueAt(json, "int")), Error);
 }
 
+TEST(optionalValueAt, existing) {
+    auto json = R"({ "string": "ssh-rsa" })"_json;
+
+    ASSERT_EQ(optionalValueAt(json, "string"), std::optional { "ssh-rsa" });
+}
+
+TEST(optionalValueAt, empty) {
+    auto json = R"({})"_json;
+
+    ASSERT_EQ(optionalValueAt(json, "string2"), std::nullopt);
+}
+
 } /* namespace nix */