diff --git a/lib/ldclient-rb/ldclient.rb b/lib/ldclient-rb/ldclient.rb
index 37d80e9a..d96dd1f7 100644
--- a/lib/ldclient-rb/ldclient.rb
+++ b/lib/ldclient-rb/ldclient.rb
@@ -407,6 +407,12 @@ def evaluate_internal(key, user, default, event_factory)
         return detail
       end
 
+      if user[:key].nil?
+        @config.logger.warn { "[LDClient] Variation called with nil user key; returning default value" }
+        detail = Evaluator.error_result(EvaluationReason::ERROR_USER_NOT_SPECIFIED, default)
+        return detail
+      end
+
       if !initialized?
         if @store.initialized?
           @config.logger.warn { "[LDClient] Client has not finished initializing; using last known values from feature store" }
diff --git a/spec/ldclient_spec.rb b/spec/ldclient_spec.rb
index cad4c03c..8e2ef650 100644
--- a/spec/ldclient_spec.rb
+++ b/spec/ldclient_spec.rb
@@ -171,20 +171,12 @@ def event_processor
       client.variation("key", user_anonymous, "default")
     end
 
-    it "queues a feature event for an existing feature when user key is nil" do
+    it "does not queue a feature event for an existing feature when user key is nil" do
       config.feature_store.init({ LaunchDarkly::FEATURES => {} })
       config.feature_store.upsert(LaunchDarkly::FEATURES, feature_with_value)
       bad_user = { name: "Bob" }
-      expect(event_processor).to receive(:add_event).with(hash_including(
-        kind: "feature",
-        key: "key",
-        version: 100,
-        user: bad_user,
-        value: "default",
-        default: "default",
-        trackEvents: true,
-        debugEventsUntilDate: 1000
-      ))
+      expect(event_processor).not_to receive(:add_event)
+      expect(logger).to receive(:warn)
       client.variation("key", bad_user, "default")
     end