diff --git a/src/libfetchers/fetchers.cc b/src/libfetchers/fetchers.cc
index a06d931db..0577b8d9d 100644
--- a/src/libfetchers/fetchers.cc
+++ b/src/libfetchers/fetchers.cc
@@ -419,9 +419,13 @@ 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) };
+    fetchers::PublicKey res = {  };
+    if (auto type = optionalValueAt(json, "type"))
+        res.type = getString(*type);
+
+    res.key = getString(valueAt(json, "key"));
+
+    return res;
 }
 
 void adl_serializer<fetchers::PublicKey>::to_json(json & json, fetchers::PublicKey p) {
diff --git a/tests/unit/libfetchers/data/public-key/defaultType.json b/tests/unit/libfetchers/data/public-key/defaultType.json
new file mode 100644
index 000000000..43f02a420
--- /dev/null
+++ b/tests/unit/libfetchers/data/public-key/defaultType.json
@@ -0,0 +1,4 @@
+{
+  "key": "ABCDE",
+  "type": "ssh-ed25519"
+}
diff --git a/tests/unit/libfetchers/data/public-key/simple.json b/tests/unit/libfetchers/data/public-key/simple.json
new file mode 100644
index 000000000..f83b927ac
--- /dev/null
+++ b/tests/unit/libfetchers/data/public-key/simple.json
@@ -0,0 +1,4 @@
+{
+  "key": "ABCDE",
+  "type": "ssh-rsa"
+}
diff --git a/tests/unit/libfetchers/public-key.cc b/tests/unit/libfetchers/public-key.cc
index fcd5c3af0..941ae9db7 100644
--- a/tests/unit/libfetchers/public-key.cc
+++ b/tests/unit/libfetchers/public-key.cc
@@ -1,18 +1,45 @@
 #include <gtest/gtest.h>
 #include "fetchers.hh"
 #include "json-utils.hh"
+#include <nlohmann/json.hpp>
+#include "tests/characterization.hh"
 
 namespace nix {
-    TEST(PublicKey, jsonSerialization) {
-        auto json = nlohmann::json(fetchers::PublicKey { .key = "ABCDE" });
 
-        ASSERT_EQ(json, R"({ "key": "ABCDE", "type": "ssh-ed25519" })"_json);
-    }
-    TEST(PublicKey, jsonDeserialization) {
-        auto pubKeyJson = R"({ "key": "ABCDE", "type": "ssh-ed25519" })"_json;
-        fetchers::PublicKey pubKey = pubKeyJson;
+using nlohmann::json;
 
-        ASSERT_EQ(pubKey.key, "ABCDE");
-        ASSERT_EQ(pubKey.type, "ssh-ed25519");
+class PublicKeyTest : public CharacterizationTest
+{
+    Path unitTestData = getUnitTestData() + "/public-key";
+
+public:
+    Path goldenMaster(std::string_view testStem) const override {
+        return unitTestData + "/" + testStem;
     }
+};
+
+#define TEST_JSON(FIXTURE, NAME, VAL)                                     \
+    TEST_F(FIXTURE, PublicKey_ ## NAME ## _from_json) {                   \
+        readTest(#NAME ".json", [&](const auto & encoded_) {              \
+            fetchers::PublicKey expected { VAL };                         \
+            auto got = nlohmann::json::parse(encoded_);                   \
+            ASSERT_EQ(got, expected);                                     \
+        });                                                               \
+    }                                                                     \
+                                                                          \
+    TEST_F(FIXTURE, PublicKey_ ## NAME ## _to_json) {                     \
+        writeTest(#NAME ".json", [&]() -> json {                          \
+            return nlohmann::json(fetchers::PublicKey { VAL });           \
+        }, [](const auto & file) {                                        \
+            return json::parse(readFile(file));                           \
+        }, [](const auto & file, const auto & got) {                      \
+            return writeFile(file, got.dump(2) + "\n");                   \
+        });                                                               \
+    }
+
+TEST_JSON(PublicKeyTest, simple, (fetchers::PublicKey { .type = "ssh-rsa", .key = "ABCDE" }))
+
+TEST_JSON(PublicKeyTest, defaultType, fetchers::PublicKey { .key = "ABCDE" })
+
+#undef TEST_JSON
 }
diff --git a/tests/unit/libutil/json-utils.cc b/tests/unit/libutil/json-utils.cc
index ec653fff5..c9370a74b 100644
--- a/tests/unit/libutil/json-utils.cc
+++ b/tests/unit/libutil/json-utils.cc
@@ -169,7 +169,19 @@ TEST(optionalValueAt, existing) {
 TEST(optionalValueAt, empty) {
     auto json = R"({})"_json;
 
-    ASSERT_EQ(optionalValueAt(json, "string2"), std::nullopt);
+    ASSERT_EQ(optionalValueAt(json, "string"), std::nullopt);
+}
+
+TEST(getNullable, null) {
+    auto json = R"(null)"_json;
+
+    ASSERT_EQ(getNullable(json), std::nullopt);
+}
+
+TEST(getNullable, empty) {
+    auto json = R"({})"_json;
+
+    ASSERT_EQ(getNullable(json), std::optional { R"({})"_json });
 }
 
 } /* namespace nix */