diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 85dce768cf3..0af60645545 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -183,7 +183,7 @@ class MergeRequest < ActiveRecord::Base
   def diff_base_commit
     if merge_request_diff
       merge_request_diff.base_commit
-    else
+    elsif source_sha
       self.target_project.merge_base_commit(self.source_sha, self.target_branch)
     end
   end
@@ -489,7 +489,7 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def source_sha
-    last_commit.sha
+    last_commit.try(:sha)
   end
 
   def fetch_ref
diff --git a/app/models/project.rb b/app/models/project.rb
index 488dc98c17f..238932f59a7 100644
--- a/app/models/project.rb
+++ b/app/models/project.rb
@@ -349,7 +349,8 @@ class Project < ActiveRecord::Base
   end
 
   def merge_base_commit(first_commit_id, second_commit_id)
-    repository.commit(repository.merge_base(first_commit_id, second_commit_id))
+    sha = repository.merge_base(first_commit_id, second_commit_id)
+    repository.commit(sha) if sha
   end
 
   def saved?
diff --git a/app/models/repository.rb b/app/models/repository.rb
index 6c1ee4b29cd..130daddd9d1 100644
--- a/app/models/repository.rb
+++ b/app/models/repository.rb
@@ -589,6 +589,8 @@ class Repository
 
   def merge_base(first_commit_id, second_commit_id)
     rugged.merge_base(first_commit_id, second_commit_id)
+  rescue Rugged::ReferenceError
+    nil
   end
 
   def is_ancestor?(ancestor_id, descendant_id)
diff --git a/spec/models/build_spec.rb b/spec/models/build_spec.rb
index d30bc7d0554..606340d87e4 100644
--- a/spec/models/build_spec.rb
+++ b/spec/models/build_spec.rb
@@ -1,7 +1,7 @@
 require 'spec_helper'
 
 describe Ci::Build, models: true do
-  let(:project) { FactoryGirl.create :empty_project }
+  let(:project) { FactoryGirl.create :project }
   let(:commit) { FactoryGirl.create :ci_commit, project: project }
   let(:build) { FactoryGirl.create :ci_build, commit: commit }