diff --git a/lib/optimizely.rb b/lib/optimizely.rb
index fe8997a9..8750fbac 100644
--- a/lib/optimizely.rb
+++ b/lib/optimizely.rb
@@ -96,15 +96,11 @@ def activate(experiment_key, user_id, attributes = nil)
 
       return nil unless Optimizely::Helpers::Validator.inputs_valid?(
         {
-          experiment_key: experiment_key
+          experiment_key: experiment_key,
+          user_id: user_id
         }, @logger, Logger::ERROR
       )
 
-      unless user_id.is_a?(String)
-        @logger.log(Logger::ERROR, "#{Optimizely::Helpers::Constants::INPUT_VARIABLES['USER_ID']} is invalid")
-        return nil
-      end
-
       variation_key = get_variation(experiment_key, user_id, attributes)
 
       if variation_key.nil?
@@ -136,15 +132,11 @@ def get_variation(experiment_key, user_id, attributes = nil)
 
       return nil unless Optimizely::Helpers::Validator.inputs_valid?(
         {
-          experiment_key: experiment_key
+          experiment_key: experiment_key,
+          user_id: user_id
         }, @logger, Logger::ERROR
       )
 
-      unless user_id.is_a?(String)
-        @logger.log(Logger::ERROR, "#{Optimizely::Helpers::Constants::INPUT_VARIABLES['USER_ID']} is invalid")
-        return nil
-      end
-
       unless user_inputs_valid?(attributes)
         @logger.log(Logger::INFO, "Not activating user '#{user_id}.")
         return nil
@@ -202,15 +194,11 @@ def track(event_key, user_id, attributes = nil, event_tags = nil)
 
       return nil unless Optimizely::Helpers::Validator.inputs_valid?(
         {
-          event_key: event_key
+          event_key: event_key,
+          user_id: user_id
         }, @logger, Logger::ERROR
       )
 
-      unless user_id.is_a?(String)
-        @logger.log(Logger::ERROR, "#{Optimizely::Helpers::Constants::INPUT_VARIABLES['USER_ID']} is invalid")
-        return nil
-      end
-
       return nil unless user_inputs_valid?(attributes, event_tags)
 
       experiment_ids = @config.get_experiment_ids_for_event(event_key)
@@ -265,15 +253,11 @@ def is_feature_enabled(feature_flag_key, user_id, attributes = nil)
 
       return false unless Optimizely::Helpers::Validator.inputs_valid?(
         {
-          feature_flag_key: feature_flag_key
+          feature_flag_key: feature_flag_key,
+          user_id: user_id
         }, @logger, Logger::ERROR
       )
 
-      unless user_id.is_a?(String)
-        @logger.log(Logger::ERROR, "#{Optimizely::Helpers::Constants::INPUT_VARIABLES['USER_ID']} is invalid")
-        return false
-      end
-
       return false unless user_inputs_valid?(attributes)
 
       feature_flag = @config.get_feature_flag_from_key(feature_flag_key)
@@ -323,10 +307,11 @@ def get_enabled_features(user_id, attributes = nil)
         return enabled_features
       end
 
-      unless user_id.is_a?(String)
-        @logger.log(Logger::ERROR, "#{Optimizely::Helpers::Constants::INPUT_VARIABLES['USER_ID']} is invalid")
-        return enabled_features
-      end
+      return enabled_features unless Optimizely::Helpers::Validator.inputs_valid?(
+        {
+          user_id: user_id
+        }, @logger, Logger::ERROR
+      )
 
       return enabled_features unless user_inputs_valid?(attributes)
 
@@ -467,16 +452,12 @@ def get_feature_variable_for_type(feature_flag_key, variable_key, variable_type,
         {
           feature_flag_key: feature_flag_key,
           variable_key: variable_key,
+          user_id: user_id,
           variable_type: variable_type
         },
         @logger, Logger::ERROR
       )
 
-      unless user_id.is_a?(String)
-        @logger.log(Logger::ERROR, "#{Optimizely::Helpers::Constants::INPUT_VARIABLES['USER_ID']} is invalid")
-        return nil
-      end
-
       return nil unless user_inputs_valid?(attributes)
 
       feature_flag = @config.get_feature_flag_from_key(feature_flag_key)
diff --git a/lib/optimizely/helpers/validator.rb b/lib/optimizely/helpers/validator.rb
index 041e31aa..3a8e0af1 100644
--- a/lib/optimizely/helpers/validator.rb
+++ b/lib/optimizely/helpers/validator.rb
@@ -108,13 +108,21 @@ def inputs_valid?(variables, logger = NoOpLogger.new, level = Logger::ERROR)
         return false unless variables.respond_to?(:each) && !variables.empty?
 
         is_valid = true
+        if variables.include? :user_id
+          unless variables[:user_id].is_a?(String)
+            is_valid = false
+            logger.log(level, "#{Constants::INPUT_VARIABLES['USER_ID']} is invalid")
+          end
+          variables.delete :user_id
+        end
+
         variables.each do |key, value|
           next if value.is_a?(String) && !value.empty?
 
           is_valid = false
           next unless logger_valid?(logger) && level
 
-          logger.log(level, "#{Optimizely::Helpers::Constants::INPUT_VARIABLES[key.to_s.upcase]} is invalid")
+          logger.log(level, "#{Constants::INPUT_VARIABLES[key.to_s.upcase]} is invalid")
         end
         is_valid
       end
diff --git a/lib/optimizely/project_config.rb b/lib/optimizely/project_config.rb
index fb178800..3a90c6be 100644
--- a/lib/optimizely/project_config.rb
+++ b/lib/optimizely/project_config.rb
@@ -282,15 +282,11 @@ def get_forced_variation(experiment_key, user_id)
 
       return nil unless Optimizely::Helpers::Validator.inputs_valid?(
         {
-          experiment_key: experiment_key
+          experiment_key: experiment_key,
+          user_id: user_id
         }, @logger, Logger::DEBUG
       )
 
-      unless user_id.is_a?(String)
-        @logger.log(Logger::DEBUG, "#{Optimizely::Helpers::Constants::INPUT_VARIABLES['USER_ID']} is invalid")
-        return nil
-      end
-
       unless @forced_variation_map.key? user_id
         @logger.log(Logger::DEBUG, "User '#{user_id}' is not in the forced variation map.")
         return nil
@@ -333,15 +329,10 @@ def set_forced_variation(experiment_key, user_id, variation_key)
       #
       # Returns a boolean value that indicates if the set completed successfully.
 
-      input_values = {experiment_key: experiment_key}
+      input_values = {experiment_key: experiment_key, user_id: user_id}
       input_values[:variation_key] = variation_key unless variation_key.nil?
       return false unless Optimizely::Helpers::Validator.inputs_valid?(input_values, @logger, Logger::DEBUG)
 
-      unless user_id.is_a?(String)
-        @logger.log(Logger::DEBUG, "#{Optimizely::Helpers::Constants::INPUT_VARIABLES['USER_ID']} is invalid")
-        return false
-      end
-
       experiment = get_experiment_from_key(experiment_key)
       experiment_id = experiment['id'] if experiment
       #  check if the experiment exists in the datafile
diff --git a/spec/project_config_spec.rb b/spec/project_config_spec.rb
index 7116de54..50b341d0 100644
--- a/spec/project_config_spec.rb
+++ b/spec/project_config_spec.rb
@@ -947,6 +947,7 @@
       expect(Optimizely::Helpers::Validator).to receive(:inputs_valid?).with(
         {
           experiment_key: @valid_experiment[:key],
+          user_id: @user_id,
           variation_key: @valid_variation[:key]
         }, spy_logger, Logger::DEBUG
       )
@@ -966,7 +967,8 @@
     it 'should call inputs_valid? with the proper arguments in get_forced_variation' do
       expect(Optimizely::Helpers::Validator).to receive(:inputs_valid?).with(
         {
-          experiment_key: @valid_experiment[:key]
+          experiment_key: @valid_experiment[:key],
+          user_id: @user_id
         }, spy_logger, Logger::DEBUG
       )
       config.get_forced_variation(@valid_experiment[:key], @user_id)
diff --git a/spec/project_spec.rb b/spec/project_spec.rb
index f6300088..b47a08d5 100644
--- a/spec/project_spec.rb
+++ b/spec/project_spec.rb
@@ -271,7 +271,8 @@ class InvalidErrorHandler; end
     it 'should call inputs_valid? with the proper arguments in activate' do
       expect(Optimizely::Helpers::Validator).to receive(:inputs_valid?).with(
         {
-          experiment_key: 'test_experiment_with_audience'
+          experiment_key: 'test_experiment_with_audience',
+          user_id: 'test_user'
         }, spy_logger, Logger::ERROR
       )
       project_instance.activate('test_experiment_with_audience', 'test_user')
@@ -454,7 +455,8 @@ class InvalidErrorHandler; end
     it 'should call inputs_valid? with the proper arguments in track' do
       expect(Optimizely::Helpers::Validator).to receive(:inputs_valid?).with(
         {
-          event_key: 'test_event'
+          event_key: 'test_event',
+          user_id: 'test_user'
         }, spy_logger, Logger::ERROR
       )
       project_instance.track('test_event', 'test_user')
@@ -646,7 +648,8 @@ class InvalidErrorHandler; end
     it 'should call inputs_valid? with the proper arguments in get_variation' do
       expect(Optimizely::Helpers::Validator).to receive(:inputs_valid?).with(
         {
-          experiment_key: 'test_experiment_with_audience'
+          experiment_key: 'test_experiment_with_audience',
+          user_id: 'test_user'
         }, spy_logger, Logger::ERROR
       )
       project_instance.get_variation('test_experiment_with_audience', 'test_user', nil)
@@ -799,7 +802,8 @@ class InvalidErrorHandler; end
     it 'should call inputs_valid? with the proper arguments in is_feature_enabled' do
       expect(Optimizely::Helpers::Validator).to receive(:inputs_valid?).with(
         {
-          feature_flag_key: 'multi_variate_feature'
+          feature_flag_key: 'multi_variate_feature',
+          user_id: 'test_user'
         }, spy_logger, Logger::ERROR
       )
       project_instance.is_feature_enabled('multi_variate_feature', 'test_user')
@@ -926,6 +930,15 @@ class InvalidErrorHandler; end
       expect(invalid_project.get_enabled_features('test_user')).to be_empty
     end
 
+    it 'should call inputs_valid? with the proper arguments in get_enabled_features' do
+      expect(Optimizely::Helpers::Validator).to receive(:inputs_valid?).with(
+        {
+          user_id: 'test_user'
+        }, spy_logger, Logger::ERROR
+      )
+      project_instance.get_enabled_features('test_user')
+    end
+
     it 'should return empty when no feature flag is enabled' do
       allow(project_instance).to receive(:is_feature_enabled).and_return(false)
       expect(project_instance.get_enabled_features('test_user')).to be_empty
@@ -1249,11 +1262,11 @@ class InvalidErrorHandler; end
         {
           feature_flag_key: 'integer_single_variable_feature',
           variable_key: 'integer_variable',
+          user_id: 'test_user',
           variable_type: 'integer'
         }, spy_logger, Logger::ERROR
       )
-      expect(project_instance.get_feature_variable_integer('integer_single_variable_feature', 'integer_variable', nil, user_attributes))
-        .to eq(nil)
+      project_instance.get_feature_variable_integer('integer_single_variable_feature', 'integer_variable', 'test_user', user_attributes)
     end
 
     it 'should log and return nil when user ID is non string' do