diff --git a/app/models/exercise.rb b/app/models/exercise.rb
index 5f9a927a..d0aa9605 100644
--- a/app/models/exercise.rb
+++ b/app/models/exercise.rb
@@ -372,11 +372,11 @@ class Exercise < ApplicationRecord
   end
 
   def retrieve_working_time_statistics
-    @working_time_statistics = {}
+    @working_time_statistics = {'InternalUser' => {}, 'ExternalUser' => {}}
     ActiveRecord::Base.transaction do
       self.class.connection.execute("SET LOCAL intervalstyle = 'postgres'")
       self.class.connection.execute(user_working_time_query).each do |tuple|
-        @working_time_statistics[tuple['user_id'].to_i] = tuple
+        @working_time_statistics[tuple['user_type']][tuple['user_id'].to_i] = tuple
       end
     end
   end
@@ -392,9 +392,9 @@ class Exercise < ApplicationRecord
     end
   end
 
-  def average_working_time_for(user_id)
+  def average_working_time_for(user)
     retrieve_working_time_statistics if @working_time_statistics.nil?
-    @working_time_statistics[user_id]['working_time']
+    @working_time_statistics[user.class.name][user.id]['working_time']
   end
 
   def accumulated_working_time_for_only(user)
diff --git a/app/views/exercises/feedback.html.slim b/app/views/exercises/feedback.html.slim
index 0aee4422..aa06a509 100644
--- a/app/views/exercises/feedback.html.slim
+++ b/app/views/exercises/feedback.html.slim
@@ -32,7 +32,7 @@ h1 = link_to_if(policy(@exercise).show?, @exercise, exercise_path(@exercise))
           .card-footer
             div.clearfix.feedback-header
               span.points.flex-grow-1 = "#{t('exercises.statistics.score')}: #{@submissions[index].score}"
-              span.working_time.pull-right = "#{t('exercises.statistics.worktime')}: #{@exercise.average_working_time_for(feedback.user.id) or 0}"
+              span.working_time.pull-right = "#{t('exercises.statistics.worktime')}: #{@exercise.average_working_time_for(feedback.user) or 0}"
 
   = render('shared/pagination', collection: @feedbacks)
 
diff --git a/app/views/exercises/statistics.html.slim b/app/views/exercises/statistics.html.slim
index f5ec9734..afb78b3e 100644
--- a/app/views/exercises/statistics.html.slim
+++ b/app/views/exercises/statistics.html.slim
@@ -48,7 +48,7 @@ h1 = @exercise
   - if symbol==:external_users
     - working_time_array = []
     - @exercise.send(symbol).distinct().each do |user|
-      - working_time = @exercise.average_working_time_for(user.id) or 0
+      - working_time = @exercise.average_working_time_for(user) or 0
       - working_time_array.push working_time
     hr
     .d-none#data data-working-time=ActiveSupport::JSON.encode(working_time_array)
@@ -88,4 +88,4 @@ h1 = @exercise
                   - elsif latest_user_submission.after_late_deadline?
                     .unit-test-result.negative-result.after_late_deadline
               td = us['runs'] if policy(@exercise).detailed_statistics?
-              td = @exercise.average_working_time_for(user.id) or 0 if policy(@exercise).detailed_statistics?
+              td = @exercise.average_working_time_for(user) or 0 if policy(@exercise).detailed_statistics?