diff --git a/bundler/lib/bundler/compact_index_client/updater.rb b/bundler/lib/bundler/compact_index_client/updater.rb
index 1dc70012c288..9348f3f5d082 100644
--- a/bundler/lib/bundler/compact_index_client/updater.rb
+++ b/bundler/lib/bundler/compact_index_client/updater.rb
@@ -58,8 +58,8 @@ def update(local_path, remote_path, retrying = nil)
             end
           end
 
-          etag = (response["ETag"] || "").gsub(%r{\AW/}, "")
-          if etag.length.zero? || etag_for(local_temp_path) == etag
+          response_etag = (response["ETag"] || "").gsub(%r{\AW/}, "")
+          if etag_for(local_temp_path) == response_etag
             SharedHelpers.filesystem_access(local_path) do
               FileUtils.mv(local_temp_path, local_path)
             end
@@ -67,7 +67,7 @@ def update(local_path, remote_path, retrying = nil)
           end
 
           if retrying
-            raise MisMatchedChecksumError.new(remote_path, etag, etag_for(local_temp_path))
+            raise MisMatchedChecksumError.new(remote_path, response_etag, etag_for(local_temp_path))
           end
 
           update(local_path, remote_path, :retrying)
diff --git a/bundler/spec/bundler/compact_index_client/updater_spec.rb b/bundler/spec/bundler/compact_index_client/updater_spec.rb
index 4acd7dbc6341..699ce684d839 100644
--- a/bundler/spec/bundler/compact_index_client/updater_spec.rb
+++ b/bundler/spec/bundler/compact_index_client/updater_spec.rb
@@ -16,11 +16,14 @@
     # Regression test for https://github.com/rubygems/bundler/issues/5463
     let(:response) { double(:response, :body => "abc123") }
 
-    it "treats the response as an update" do
-      expect(response).to receive(:[]).with("ETag") { nil }
-      expect(fetcher).to receive(:call) { response }
+    it "MisMatchedChecksumError is raised" do
+      # Twice: #update retries on failure
+      expect(response).to receive(:[]).with("ETag").twice { nil }
+      expect(fetcher).to receive(:call).twice { response }
 
-      updater.update(local_path, remote_path)
+      expect do
+        updater.update(local_path, remote_path)
+      end.to raise_error(Bundler::CompactIndexClient::Updater::MisMatchedChecksumError)
     end
   end
 
@@ -56,7 +59,7 @@
       begin
         $VERBOSE = false
         Encoding.default_internal = "ASCII"
-        expect(response).to receive(:[]).with("ETag") { nil }
+        expect(response).to receive(:[]).with("ETag").and_return('"348dd9e974848a61e8abe2dffcfcce93"')
         expect(fetcher).to receive(:call) { response }
 
         updater.update(local_path, remote_path)
diff --git a/bundler/spec/install/gems/compact_index_spec.rb b/bundler/spec/install/gems/compact_index_spec.rb
index b1f361ebec94..9454508e41e4 100644
--- a/bundler/spec/install/gems/compact_index_spec.rb
+++ b/bundler/spec/install/gems/compact_index_spec.rb
@@ -826,6 +826,28 @@ def start
     expect(the_bundle).to include_gems "rack 1.0.0"
   end
 
+  it "performs full update if server endpoints serve partial content responses but don't have incremental content and provide no Etag" do
+    build_repo4 do
+      build_gem "rack", "0.9.1"
+    end
+
+    install_gemfile <<-G, :artifice => "compact_index", :env => { "BUNDLER_SPEC_GEM_REPO" => gem_repo4.to_s }
+      source "#{source_uri}"
+      gem 'rack', '0.9.1'
+    G
+
+    update_repo4 do
+      build_gem "rack", "1.0.0"
+    end
+
+    install_gemfile <<-G, :artifice => "compact_index_partial_update_no_etag_not_incremental", :env => { "BUNDLER_SPEC_GEM_REPO" => gem_repo4.to_s }
+      source "#{source_uri}"
+      gem 'rack', '1.0.0'
+    G
+
+    expect(the_bundle).to include_gems "rack 1.0.0"
+  end
+
   it "performs full update of compact index info cache if range is not satisfiable" do
     gemfile <<-G
       source "#{source_uri}"
diff --git a/bundler/spec/support/artifice/compact_index_partial_update_no_etag_not_incremental.rb b/bundler/spec/support/artifice/compact_index_partial_update_no_etag_not_incremental.rb
new file mode 100644
index 000000000000..e2fa6b41386d
--- /dev/null
+++ b/bundler/spec/support/artifice/compact_index_partial_update_no_etag_not_incremental.rb
@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+require_relative "compact_index"
+
+Artifice.deactivate
+
+class CompactIndexPartialUpdateNoEtagNotIncremental < CompactIndexAPI
+  def partial_update_no_etag
+    response_body = yield
+    headers "Surrogate-Control" => "max-age=2592000, stale-while-revalidate=60"
+    content_type "text/plain"
+    requested_range_for(response_body)
+  end
+
+  get "/versions" do
+    partial_update_no_etag do
+      file = tmp("versions.list")
+      FileUtils.rm_f(file)
+      file = CompactIndex::VersionsFile.new(file.to_s)
+      file.create(gems)
+      lines = file.contents.split("\n")
+
+      # shuffle versions so new versions are not appended to the end
+      [*lines[0..-2], lines.last.split(",").reverse.join(",")].join("\n")
+    end
+  end
+
+  get "/info/:name" do
+    partial_update_no_etag do
+      gem = gems.find {|g| g.name == params[:name] }
+      lines = CompactIndex.info(gem ? gem.versions : []).split("\n")
+
+      # shuffle versions so new versions are not appended to the end
+      [lines.first, lines.last, *lines[1..-2]].join("\n")
+    end
+  end
+end
+
+Artifice.activate_with(CompactIndexPartialUpdateNoEtagNotIncremental)