diff --git a/app/jobs/process_uploaded_file_job.rb b/app/jobs/process_uploaded_file_job.rb
index 45984fdf0..779a9c853 100644
--- a/app/jobs/process_uploaded_file_job.rb
+++ b/app/jobs/process_uploaded_file_job.rb
@@ -88,15 +88,9 @@ def count_common_path_components(archive)
   end
 
   def count_common_elements(arrays)
-    common = []
-    loop do
-      elements = arrays.filter_map(&:shift).uniq
-      if elements.count == 1
-        common << elements[0]
-      else
-        break
-      end
-    end
-    common.count
+    return 0 if arrays.empty?
+    first = arrays.shift
+    zip = first.zip(*arrays)
+    zip.count { |x| x.uniq.count == 1 }
   end
 end
diff --git a/spec/jobs/process_uploaded_file_job_spec.rb b/spec/jobs/process_uploaded_file_job_spec.rb
index 310b86e52..1a0a20285 100644
--- a/spec/jobs/process_uploaded_file_job_spec.rb
+++ b/spec/jobs/process_uploaded_file_job_spec.rb
@@ -23,6 +23,20 @@
       ])).to eq 2
     end
 
+    it "returns correct count where all prefixes are the same" do
+      expect(job.send(:count_common_elements, [
+        ["root", "sub", "folder"],
+        ["root", "sub", "folder"]
+      ])).to eq 3
+    end
+
+    it "returns correct count where a common folder has a subfolder" do
+      expect(job.send(:count_common_elements, [
+        ["root", "sub"],
+        ["root", "sub", "folder2"]
+      ])).to eq 2
+    end
+
     it "returns zero for *some* common prefixes but not on everything" do
       expect(job.send(:count_common_elements, [
         ["folder1", "sub1"],
@@ -154,12 +168,12 @@
         Zip::File.open(file, create: true) do |zipfile|
           zipfile.mkdir("sub")
           zipfile.mkdir("sub/folder")
-          zipfile.get_output_stream("sub/folder/test.stl") { |f| f.puts "solid" }
+          zipfile.get_output_stream("sub/test.stl") { |f| f.puts "solid" }
           zipfile.get_output_stream("sub/folder/test2.stl") { |f| f.puts "solid" }
         end
         described_class.new.send(:unzip, model, Rack::Test::UploadedFile.new(file))
         expect(model.model_files.count).to eq 2
-        expect(model.model_files.map(&:filename).sort).to eq ["test.stl", "test2.stl"]
+        expect(model.model_files.map(&:filename).sort).to eq ["folder/test2.stl", "test.stl"]
       end
     end