Catch nil input for querying machine index

lib/vagrant/machine_index.rb

@@ -263,6 +263,7 @@ module Vagrant
     #
     # @return [Hash]
     def find_by_prefix(prefix)
+      return nil if prefix.nil?
       @machines.each do |uuid, data|
         return data.merge("id" => uuid) if uuid.start_with?(prefix)
       end

test/unit/vagrant/machine_index_test.rb

@@ -105,6 +105,7 @@ describe Vagrant::MachineIndex do
 
     it "returns nil if the machine doesn't exist" do
       expect(subject.get("foo")).to be_nil
+      expect(subject.get(nil)).to be_nil
     end
 
     it "returns a valid entry if the machine exists" do
@@ -139,6 +140,11 @@ describe Vagrant::MachineIndex do
       expect(subject.include?("b")).to be(true)
     end
 
+    it "should return false if given nil input" do
+      expect(subject.include?(nil)).to be(false)
+    end
+
+
     it "locks the entry so subsequent gets fail" do
       result = subject.get("bar")
       expect(result).to_not be_nil