diff --git a/lib/bundler/audit/cli.rb b/lib/bundler/audit/cli.rb
index 97979ecc..466160d2 100644
--- a/lib/bundler/audit/cli.rb
+++ b/lib/bundler/audit/cli.rb
@@ -71,6 +71,9 @@ def check(dir=Dir.pwd)
                    rescue Bundler::GemfileLockNotFound => exception
                      say exception.message, :red
                      exit 1
+                   rescue Scanner::InvalidGemfileLock => exception
+                     say_error exception.message, :red
+                     exit 1
                    end
 
         report = scanner.report(ignore: options.ignore)
diff --git a/lib/bundler/audit/scanner.rb b/lib/bundler/audit/scanner.rb
index f0e79a30..1613931c 100644
--- a/lib/bundler/audit/scanner.rb
+++ b/lib/bundler/audit/scanner.rb
@@ -36,6 +36,12 @@ module Audit
     #
     class Scanner
 
+      # Raised when the given file is not a valid Bundler lockfile.
+      class InvalidGemfileLock < StandardError; end
+
+      # Regexp matching the first line of every valid Bundler lockfile.
+      LOCKFILE_HEADER_RE = /\A(GEM|GIT|PATH|PLUGIN SOURCE|PLATFORMS|DEPENDENCIES|BUNDLED WITH)\b/
+
       # The advisory database.
       #
       # @return [Database]
@@ -84,7 +90,13 @@ def initialize(root=Dir.pwd,gemfile_lock='Gemfile.lock',database=Database.new,co
           raise(Bundler::GemfileLockNotFound,"Could not find #{gemfile_lock.inspect} in #{@root.inspect}")
         end
 
-        @lockfile = LockfileParser.new(File.read(gemfile_lock_path))
+        lock_file_content = File.read(gemfile_lock_path)
+
+        unless lock_file_content =~ LOCKFILE_HEADER_RE
+          raise(InvalidGemfileLock,"#{gemfile_lock.inspect} is not a valid Gemfile.lock")
+        end
+
+        @lockfile = LockfileParser.new(lock_file_content)
 
         config_dot_file_full_path = File.absolute_path(config_dot_file, @root)
 
diff --git a/spec/cli_spec.rb b/spec/cli_spec.rb
index 11e4f256..0d1de922 100644
--- a/spec/cli_spec.rb
+++ b/spec/cli_spec.rb
@@ -161,4 +161,35 @@
       end
     end
   end
+
+  describe "#check" do
+    context "when --gemfile-lock is not a valid lock file" do
+      let(:bundle_dir)    { File.expand_path(File.join('spec','bundle','unpatched_gems')) }
+      let(:database_path) { Fixtures::Database::PATH }
+
+      it "must print an error message to stderr" do
+        expect {
+          begin
+            described_class.start ['check', bundle_dir,
+                                   '--gemfile-lock', 'Gemfile',
+                                   '--database', database_path,
+                                   '--no-update']
+          rescue SystemExit
+          end
+        }.to output(/is not a valid Gemfile\.lock/).to_stderr
+      end
+
+      it "must exit with status 1" do
+        expect {
+          described_class.start ['check', bundle_dir,
+                                 '--gemfile-lock', 'Gemfile',
+                                 '--database', database_path,
+                                 '--no-update']
+        }.to raise_error(SystemExit) do |error|
+          expect(error.success?).to eq(false)
+          expect(error.status).to eq(1)
+        end
+      end
+    end
+  end
 end
diff --git a/spec/scanner_spec.rb b/spec/scanner_spec.rb
index afde727c..4cbfb600 100644
--- a/spec/scanner_spec.rb
+++ b/spec/scanner_spec.rb
@@ -7,6 +7,16 @@
 
   subject { described_class.new(directory) }
 
+  describe "#initialize" do
+    context "when given a non-lock file as gemfile_lock" do
+      it "should raise InvalidGemfileLock" do
+        expect {
+          described_class.new(directory, 'Gemfile')
+        }.to raise_error(Scanner::InvalidGemfileLock,/is not a valid Gemfile\.lock/)
+      end
+    end
+  end
+
   describe "#scan" do
     it "should yield results" do
       results = []