diff --git a/lib/bundler/fetcher/compact_index.rb b/lib/bundler/fetcher/compact_index.rb
index f249ec1835a..81cd11eee0a 100644
--- a/lib/bundler/fetcher/compact_index.rb
+++ b/lib/bundler/fetcher/compact_index.rb
@@ -61,6 +61,9 @@ def fetch_spec(spec)
       def available?
         # Read info file checksums out of /versions, so we can know if gems are up to date
         fetch_uri.scheme != "file" && compact_index_client.update_and_parse_checksums!
+      rescue CompactIndexClient::Updater::MisMatchedChecksumError => e
+        Bundler.ui.warn(e.message)
+        nil
       end
       compact_index_request :available?
 
diff --git a/lib/bundler/vendor/compact_index_client/lib/compact_index_client.rb b/lib/bundler/vendor/compact_index_client/lib/compact_index_client.rb
index e7d67d99268..1679c17c8a3 100644
--- a/lib/bundler/vendor/compact_index_client/lib/compact_index_client.rb
+++ b/lib/bundler/vendor/compact_index_client/lib/compact_index_client.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require "pathname"
 require "set"
 
diff --git a/lib/bundler/vendor/compact_index_client/lib/compact_index_client/cache.rb b/lib/bundler/vendor/compact_index_client/lib/compact_index_client/cache.rb
index 57ef5518498..a370fa402d8 100644
--- a/lib/bundler/vendor/compact_index_client/lib/compact_index_client/cache.rb
+++ b/lib/bundler/vendor/compact_index_client/lib/compact_index_client/cache.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 class Bundler::CompactIndexClient
   class Cache
     attr_reader :directory
diff --git a/lib/bundler/vendor/compact_index_client/lib/compact_index_client/updater.rb b/lib/bundler/vendor/compact_index_client/lib/compact_index_client/updater.rb
index a3cdc551406..630318a9be5 100644
--- a/lib/bundler/vendor/compact_index_client/lib/compact_index_client/updater.rb
+++ b/lib/bundler/vendor/compact_index_client/lib/compact_index_client/updater.rb
@@ -1,9 +1,21 @@
+# frozen_string_literal: true
 require "stringio"
 require "zlib"
 
 class Bundler::CompactIndexClient
   class Updater
-    class MisMatchedChecksumError < Error; end
+    class MisMatchedChecksumError < Error
+      def initialize(path, server_checksum, local_checksum)
+        @path = path
+        @server_checksum = server_checksum
+        @local_checksum = local_checksum
+      end
+
+      def message
+        "The checksum of /#{@path} does not match the checksum provided by the server! Something is wrong " \
+          "(local checksum is #{@local_checksum.inspect}, was expecting #{@server_checksum.inspect})."
+      end
+    end
 
     def initialize(fetcher)
       @fetcher = fetcher
@@ -31,20 +43,20 @@ def update(local_path, remote_path, retrying = nil)
       mode = response.is_a?(Net::HTTPPartialContent) ? "a" : "w"
       local_path.open(mode) {|f| f << content }
 
-      return if etag_for(local_path) == response["ETag"]
+      response_etag = response["ETag"]
+      return if etag_for(local_path) == response_etag
 
       if retrying.nil?
         local_path.delete
         update(local_path, remote_path, :retrying)
       else
-        raise MisMatchedChecksumError, "Checksum of /#{remote_path} " \
-          "does not match the checksum provided by server! Something is wrong."
+        raise MisMatchedChecksumError.new(remote_path, response_etag, etag_for(local_path))
       end
     end
 
     def etag_for(path)
       sum = checksum_for_file(path)
-      sum ? '"' << sum << '"' : nil
+      sum ? %("#{sum}") : nil
     end
 
     def checksum_for_file(path)
diff --git a/lib/bundler/vendor/compact_index_client/lib/compact_index_client/version.rb b/lib/bundler/vendor/compact_index_client/lib/compact_index_client/version.rb
index f18c522ae89..64520daead4 100644
--- a/lib/bundler/vendor/compact_index_client/lib/compact_index_client/version.rb
+++ b/lib/bundler/vendor/compact_index_client/lib/compact_index_client/version.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 class Bundler::CompactIndexClient
-  VERSION = "0.1.0"
+  VERSION = "0.1.0".freeze
 end
diff --git a/spec/install/gems/compact_index_spec.rb b/spec/install/gems/compact_index_spec.rb
index 13645f0ae6f..c00e73523be 100644
--- a/spec/install/gems/compact_index_spec.rb
+++ b/spec/install/gems/compact_index_spec.rb
@@ -139,6 +139,20 @@
     should_be_installed "rack 1.0.0"
   end
 
+  it "falls back when the versions endpoint has a checksum mismatch" do
+    gemfile <<-G
+      source "#{source_uri}"
+      gem "rack"
+    G
+
+    bundle! :install, :verbose => true, :artifice => "compact_index_checksum_mismatch"
+    expect(out).to include("Fetching gem metadata from #{source_uri}")
+    expect(out).to include <<-'WARN'
+The checksum of /versions does not match the checksum provided by the server! Something is wrong (local checksum is "\"d41d8cd98f00b204e9800998ecf8427e\"", was expecting "\"123\"").
+    WARN
+    should_be_installed "rack 1.0.0"
+  end
+
   it "handles host redirects" do
     gemfile <<-G
       source "#{source_uri}"
diff --git a/spec/support/artifice/compact_index_checksum_mismatch.rb b/spec/support/artifice/compact_index_checksum_mismatch.rb
new file mode 100644
index 00000000000..4ac328736cf
--- /dev/null
+++ b/spec/support/artifice/compact_index_checksum_mismatch.rb
@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+require File.expand_path("../compact_index", __FILE__)
+
+Artifice.deactivate
+
+class CompactIndexChecksumMismatch < CompactIndexAPI
+  get "/versions" do
+    headers "ETag" => quote("123")
+    headers "Surrogate-Control" => "max-age=2592000, stale-while-revalidate=60"
+    content_type "text/plain"
+    body ""
+  end
+end
+
+Artifice.activate_with(CompactIndexChecksumMismatch)