diff --git a/lib/bundler/multilock/ext/dsl.rb b/lib/bundler/multilock/ext/dsl.rb
index c83a231..b29ce60 100644
--- a/lib/bundler/multilock/ext/dsl.rb
+++ b/lib/bundler/multilock/ext/dsl.rb
@@ -20,7 +20,8 @@ def evaluate(gemfile, lockfile, unlock)
             builder = new
             builder.eval_gemfile(gemfile, &Multilock.prepare_block) if Multilock.prepare_block
             builder.eval_gemfile(gemfile)
-            if (ruby_version_requirement = builder.instance_variable_get(:@ruby_version))
+            if (ruby_version_requirement = builder.instance_variable_get(:@ruby_version)) &&
+               Multilock.lockfile_definitions[lockfile]
               Multilock.lockfile_definitions[lockfile][:ruby_version_requirement] = ruby_version_requirement
             elsif (parent_lockfile = Multilock.lockfile_definitions.dig(lockfile, :parent)) &&
                   (parent_lockfile_definition = Multilock.lockfile_definitions[parent_lockfile]) &&
diff --git a/spec/bundler/multilock_spec.rb b/spec/bundler/multilock_spec.rb
index 774c4d5..1d104b1 100644
--- a/spec/bundler/multilock_spec.rb
+++ b/spec/bundler/multilock_spec.rb
@@ -934,6 +934,16 @@
     end
   end
 
+  it "doesn't error when no lockfiles are defined but ruby version is set" do
+    with_gemfile(<<~RUBY) do
+      gem "nokogiri"
+
+      ruby ">= 2.1"
+    RUBY
+      invoke_bundler("install")
+    end
+  end
+
   private
 
   def create_local_gem(name, content = "", subdirectory: true)