diff --git a/mdio/dataset.h b/mdio/dataset.h
index b5154f7..9b0c581 100644
--- a/mdio/dataset.h
+++ b/mdio/dataset.h
@@ -799,7 +799,11 @@ class Dataset {
                 }
             }
         )";
-    nlohmann::json base = nlohmann::json::parse(baseStr);
+    nlohmann::json base = nlohmann::json::parse(baseStr, nullptr, false);
+    if (base.is_discarded()) {
+      return absl::Status(absl::StatusCode::kInternal,
+                          "Failed to parse base JSON.");
+    }
     if (found >= 0) {
       base["field"] = specJson["metadata"]["dtype"][found][0];
     } else {
diff --git a/mdio/dataset_test.cc b/mdio/dataset_test.cc
index e25d813..9bf3374 100644
--- a/mdio/dataset_test.cc
+++ b/mdio/dataset_test.cc
@@ -529,4 +529,28 @@ TEST(Dataset, commitSlicedMetadata) {
 
   EXPECT_TRUE(commitRes.status().ok()) << commitRes.status();
 }
+
+TEST(Dataset, openNonExistent) {
+  auto json_vars = GetToyExample();
+
+  auto datasetRes =
+      mdio::Dataset::from_json(json_vars, "zarrs/DNE", mdio::constants::kOpen);
+  ASSERT_FALSE(datasetRes.status().ok())
+      << "Opened a non-existent dataset without error!";
+}
+
+TEST(Dataset, kCreateOverExisting) {
+  auto json_vars = GetToyExample();
+
+  auto datasetRes = mdio::Dataset::from_json(json_vars, "zarrs/acceptance",
+                                             mdio::constants::kCreateClean);
+  ASSERT_TRUE(datasetRes.status().ok()) << datasetRes.status();
+
+  datasetRes = mdio::Dataset::from_json(json_vars, "zarrs/acceptance",
+                                        mdio::constants::kCreate);
+  ASSERT_FALSE(datasetRes.status().ok())
+      << "Created a dataset over an existing "
+         "one without error!";
+}
+
 }  // namespace
diff --git a/mdio/dataset_validator.h b/mdio/dataset_validator.h
index 10f566a..99f0c14 100644
--- a/mdio/dataset_validator.h
+++ b/mdio/dataset_validator.h
@@ -43,7 +43,11 @@ bool contains(const std::unordered_set<std::string>& set,
  * InvalidArgumentError if validation fails for any reason
  */
 absl::Status validate_schema(nlohmann::json& spec /*NOLINT*/) {
-  nlohmann::json targetSchema = nlohmann::json::parse(kDatasetSchema);
+  nlohmann::json targetSchema =
+      nlohmann::json::parse(kDatasetSchema, nullptr, false);
+  if (targetSchema.is_discarded()) {
+    return absl::NotFoundError("Failed to load schema");
+  }
 
   nlohmann::json_schema::json_validator validator(
       nullptr, nlohmann::json_schema::default_string_format_check);
diff --git a/mdio/variable.h b/mdio/variable.h
index 9b8aca8..84889a2 100644
--- a/mdio/variable.h
+++ b/mdio/variable.h
@@ -455,8 +455,8 @@ Future<Variable<T, R, M>> OpenVariable(const nlohmann::json& json_store,
   // go read the attributes return json ...
   auto parse = [](const tensorstore::kvstore::ReadResult& kvs_read,
                   const ::nlohmann::json& spec) {
-    // FIXME - if attributes supplied then validate with values
-    auto attributes = nlohmann::json::parse(std::string(kvs_read.value));
+    auto attributes =
+        nlohmann::json::parse(std::string(kvs_read.value), nullptr, false);
     return attributes;
   };