diff --git a/lib/bundler/multilock.rb b/lib/bundler/multilock.rb
index 5d56274..d9ec544 100644
--- a/lib/bundler/multilock.rb
+++ b/lib/bundler/multilock.rb
@@ -70,7 +70,8 @@ def add_lockfile(lockfile = nil,
 
         parent = expand_lockfile(parent)
         if parent != Bundler.default_lockfile(force_original: true) &&
-           !lockfile_definitions.find { |definition| definition[:lockfile] == parent }
+           !lockfile_definitions.find { |definition| definition[:lockfile] == parent } &&
+           !parent.exist?
           raise ArgumentError, "Parent lockfile #{parent} is not defined"
         end
 
diff --git a/spec/bundler/multilock_spec.rb b/spec/bundler/multilock_spec.rb
index 271f283..c45ea32 100644
--- a/spec/bundler/multilock_spec.rb
+++ b/spec/bundler/multilock_spec.rb
@@ -138,6 +138,14 @@
     end
   end
 
+  it "allows externally defined parents if they exist" do
+    with_gemfile(<<~RUBY) do
+      lockfile("full", parent: Bundler.default_lockfile)
+    RUBY
+      invoke_bundler("install")
+    end
+  end
+
   it "generates custom lockfiles with varying versions" do
     with_gemfile(<<~RUBY) do
       lockfile do