diff --git a/app/services/work_packages/set_schedule_service.rb b/app/services/work_packages/set_schedule_service.rb
index df9afc55b422..2ca837047108 100644
--- a/app/services/work_packages/set_schedule_service.rb
+++ b/app/services/work_packages/set_schedule_service.rb
@@ -116,16 +116,13 @@ def reschedule_ancestor(scheduled, dependency)
   #    ancestors limits moving it. Then it is moved to the earliest date possible. This limitation is propagated transtitively
   #    to all following work packages.
   def reschedule_by_follows(scheduled, dependency)
-    delta = if dependency.follows_moved.first
-              date_rescheduling_delta(dependency.follows_moved.first.to)
-            else
-              0
-            end
-
+    delta = follows_delta(dependency)
     min_start_date = dependency.max_date_of_followed
 
     if delta.zero? && min_start_date
       reschedule_to_date(scheduled, min_start_date)
+    elsif !scheduled.start_date
+      schedule_on_missing_dates(scheduled, min_start_date)
     elsif !delta.zero?
       reschedule_by_delta(scheduled, delta, min_start_date)
     end
@@ -149,9 +146,24 @@ def reschedule_to_date(scheduled, date)
   end
 
   def reschedule_by_delta(scheduled, delta, min_start_date)
-    required_delta = [min_start_date - scheduled.start_date, [delta, 0].min].max
+    required_delta = [min_start_date - (scheduled.start_date || min_start_date), [delta, 0].min].max
 
     scheduled.start_date += required_delta
     scheduled.due_date += required_delta if scheduled.due_date
   end
+
+  # If the start_date of scheduled is nil at this point something
+  # went wrong before. So we fix it now by setting the date.
+  def schedule_on_missing_dates(scheduled, min_start_date)
+    scheduled.start_date = min_start_date
+    scheduled.due_date = scheduled.start_date + 1 if scheduled.due_date && scheduled.due_date < scheduled.start_date
+  end
+
+  def follows_delta(dependency)
+    if dependency.follows_moved.first
+      date_rescheduling_delta(dependency.follows_moved.first.to)
+    else
+      0
+    end
+  end
 end
diff --git a/spec/services/work_packages/set_schedule_service_spec.rb b/spec/services/work_packages/set_schedule_service_spec.rb
index f03e414c3c0d..eb74f276e511 100644
--- a/spec/services/work_packages/set_schedule_service_spec.rb
+++ b/spec/services/work_packages/set_schedule_service_spec.rb
@@ -233,6 +233,16 @@ def stub_follower_child(parent, start, due)
           end
         end
       end
+
+      context 'when the follower has no start date (which should not happen)' do
+        let(:follower1_start_date) { nil }
+
+        it_behaves_like 'reschedules' do
+          let(:expected) do
+            { following_work_package1 => [Date.today + 6.days, Date.today + 7.day] }
+          end
+        end
+      end
     end
 
     context 'moving forward with the follower having some space left' do
@@ -369,6 +379,20 @@ def stub_follower_child(parent, start, due)
       end
     end
 
+    context 'moving backwards with the follower having no start date (which should not happen)' do
+      let(:follower1_start_date) { nil }
+
+      before do
+        work_package.due_date = Date.today - 5.days
+      end
+
+      it_behaves_like 'reschedules' do
+        let(:expected) do
+          { following_work_package1 => [Date.today - 4.days, follower1_due_date] }
+        end
+      end
+    end
+
     context 'removing the dates on the predecessor' do
       before do
         work_package.start_date = work_package.due_date = nil