diff --git a/lib/ldclient-rb/ldclient.rb b/lib/ldclient-rb/ldclient.rb
index 6e031aaf..c34b16e6 100644
--- a/lib/ldclient-rb/ldclient.rb
+++ b/lib/ldclient-rb/ldclient.rb
@@ -134,6 +134,7 @@ def toggle?(key, user, default = false)
         @config.logger.error("[LDClient] Must specify user")
         return default
       end
+      sanitize_user(user)
 
       if @config.stream? && !@stream_processor.started?
         @stream_processor.start
@@ -176,6 +177,7 @@ def add_event(event)
     # @param [Hash] The user to register
     #
     def identify(user)
+      sanitize_user(user)
       add_event(kind: "identify", key: user[:key], user: user)
     end
 
@@ -200,6 +202,7 @@ def is_offline?
     #
     # @return [void]
     def track(event_name, user, data)
+      sanitize_user(user)
       add_event(kind: "custom", key: event_name, user: user, data: data)
     end
 
@@ -406,9 +409,15 @@ def log_exception(caller, exn)
       @config.logger.error(error)
     end
 
+    def sanitize_user(user)
+      if user[:key]
+        user[:key] = user[:key].to_s
+      end
+    end
+
     private :post_flushed_events, :add_event, :get_streamed_flag,
             :get_flag_stream, :get_flag_int, :make_request, :param_for_user,
             :match_target?, :match_user?, :match_variation?, :evaluate,
-            :create_worker, :log_timings, :log_exception
+            :create_worker, :log_timings, :log_exception, :sanitize_user
   end
 end
diff --git a/spec/fixtures/numeric_key_user.json b/spec/fixtures/numeric_key_user.json
new file mode 100644
index 00000000..2a7ec475
--- /dev/null
+++ b/spec/fixtures/numeric_key_user.json
@@ -0,0 +1,9 @@
+{
+    "key": 33,
+    "custom":{
+        "groups":[
+            "microsoft",
+            "google"
+        ]
+    }
+}
diff --git a/spec/fixtures/sanitized_numeric_key_user.json b/spec/fixtures/sanitized_numeric_key_user.json
new file mode 100644
index 00000000..874e0067
--- /dev/null
+++ b/spec/fixtures/sanitized_numeric_key_user.json
@@ -0,0 +1,9 @@
+{
+    "key": "33",
+    "custom":{
+        "groups":[
+            "microsoft",
+            "google"
+        ]
+    }
+}
diff --git a/spec/ldclient_spec.rb b/spec/ldclient_spec.rb
index 2e48969a..2ed0d545 100644
--- a/spec/ldclient_spec.rb
+++ b/spec/ldclient_spec.rb
@@ -14,6 +14,14 @@
     data = File.read(File.join("spec", "fixtures", "user.json"))
     JSON.parse(data, symbolize_names: true)
   end
+  let(:numeric_key_user) do
+    data = File.read(File.join("spec", "fixtures", "numeric_key_user.json"))
+    JSON.parse(data, symbolize_names: true)
+  end
+  let(:sanitized_numeric_key_user) do
+    data = File.read(File.join("spec", "fixtures", "sanitized_numeric_key_user.json"))
+    JSON.parse(data, symbolize_names: true)
+  end
 
   context 'user flag settings' do
     describe '#update_user_flag_setting' do
@@ -85,6 +93,10 @@
       result = client.toggle?(feature[:key], nil, "default")
       expect(result).to eq "default"
     end
+    it "sanitizes the user in the event" do
+      expect(client).to receive(:add_event).with(hash_including(user: sanitized_numeric_key_user))
+      client.toggle?(feature[:key], numeric_key_user, "default")
+    end
     it "returns value from streamed flag if available" do
       expect(client.instance_variable_get(:@config)).to receive(:stream?).and_return(true).twice
       expect(client.instance_variable_get(:@stream_processor)).to receive(:started?).and_return true
@@ -103,6 +115,28 @@
     end
   end
 
+  describe '#identify' do 
+    it "queues up an identify event" do
+      expect(client).to receive(:add_event).with(hash_including(kind: "identify", key: user[:key], user: user))
+      client.identify(user)
+    end
+    it "sanitizes the user in the event" do
+      expect(client).to receive(:add_event).with(hash_including(user: sanitized_numeric_key_user))
+      client.identify(numeric_key_user)
+    end
+  end
+
+  describe '#track' do 
+    it "queues up an custom event" do
+      expect(client).to receive(:add_event).with(hash_including(kind: "custom", key: "custom_event_name", user: user, data: 42))
+      client.track("custom_event_name", user, 42)
+    end
+    it "sanitizes the user in the event" do
+      expect(client).to receive(:add_event).with(hash_including(user: sanitized_numeric_key_user))
+      client.track("custom_event_name", numeric_key_user, nil)
+    end
+  end
+
   describe '#get_streamed_flag' do
     it "will not check the polled flag normally" do
       expect(client).to receive(:get_flag_stream).and_return true