diff --git a/lib/gtfs/source.rb b/lib/gtfs/source.rb
index a2c7818..648f913 100644
--- a/lib/gtfs/source.rb
+++ b/lib/gtfs/source.rb
@@ -1,5 +1,6 @@
 require 'tmpdir'
 require 'fileutils'
+require 'stringio'
 require 'zip'
 
 module GTFS
@@ -57,14 +58,14 @@ def entries
       Dir.entries(@tmp_dir)
     end
 
-    def raise_if_missing_source(filename)
-      file_missing = !entries.include?(filename)
-      raise InvalidSourceException.new("Missing required source file: #{filename}") if file_missing
+    def raise_if_required_source(entity)
+      raise InvalidSourceException.new("Missing required source file: #{entity.filename}") if entity.required_file?
     end
 
     ENTITIES.each do |entity|
       define_method entity.name.to_sym do
-        parse_file entity.filename do |f|
+        parse_entity entity do |f|
+          files[entity.filename] ||= f
           entity.send("parse_#{entity.name}".to_sym, f.read, options)
         end
       end
@@ -78,10 +79,12 @@ def files
       @files ||= {}
     end
 
-    def parse_file(filename)
-      raise_if_missing_source filename
-      open File.join(@tmp_dir, '/', filename), 'r:bom|utf-8' do |f|
-        files[filename] ||= yield f
+    def parse_entity(entity)
+      if entries.include?(entity.filename)
+        yield open(File.join(@tmp_dir, entity.filename), 'r:bom|utf-8')
+      else
+        raise_if_required_source entity
+        yield StringIO.new
       end
     end
   end
diff --git a/spec/fixtures/valid_gtfs_missing_optional_files.zip b/spec/fixtures/valid_gtfs_missing_optional_files.zip
new file mode 100644
index 0000000..6fab8ea
Binary files /dev/null and b/spec/fixtures/valid_gtfs_missing_optional_files.zip differ
diff --git a/spec/gtfs/source_spec.rb b/spec/gtfs/source_spec.rb
index 934b03f..e467d42 100644
--- a/spec/gtfs/source_spec.rb
+++ b/spec/gtfs/source_spec.rb
@@ -5,10 +5,14 @@
     File.expand_path(File.dirname(__FILE__) + '/../fixtures/valid_gtfs.zip')
   end
 
-  let(:source_missing_required_files) do 
+  let(:source_missing_required_files) do
     File.expand_path(File.dirname(__FILE__) + '/../fixtures/missing_files.zip')
   end
 
+  let (:valid_local_source_missing_optional_files) do
+    File.expand_path(File.dirname(__FILE__) + '/../fixtures/valid_gtfs_missing_optional_files.zip')
+  end
+
   describe '#build' do
     let(:opts) {{}}
     let(:data_source) {valid_local_source}
@@ -78,4 +82,15 @@
 
   describe '#stop_times' do
   end
+
+  describe '#transfers' do
+    subject {source.transfers}
+
+    context 'when the source is missing transfers' do
+      let(:source) { GTFS::Source.build valid_local_source_missing_optional_files }
+
+      it {should_not raise_exception GTFS::InvalidSourceException}
+      it {should be_empty}
+    end
+  end
 end