diff --git a/lib/register/instruction.rb b/lib/register/instruction.rb
index 0002d751..2123f7cf 100644
--- a/lib/register/instruction.rb
+++ b/lib/register/instruction.rb
@@ -16,8 +16,10 @@ module Register
     include Positioned
 
     def initialize source , nekst = nil
-        @source = source
-        @next = nekst
+      @source = source
+      @next = nekst
+      return unless source
+      raise "Source must be string or ast node, not #{source.class}" unless source.is_a?(String) or source.is_a?(AST::Node)
     end
     attr_reader :source
 
diff --git a/test/register/test_instructions.rb b/test/register/test_instructions.rb
index 4e286a55..d904fd61 100644
--- a/test/register/test_instructions.rb
+++ b/test/register/test_instructions.rb
@@ -3,9 +3,9 @@ require_relative "../helper"
 module Register
   class TestInstructions < MiniTest::Test
     def setup
-      @label = Label.new(nil , "test")
-      @branch = Branch.new(nil , @label)
-      @instruction = Instruction.new(nil)
+      @label = Label.new("test" , "test")
+      @branch = Branch.new("test" , @label)
+      @instruction = Instruction.new("test")
     end
     def test_branch_tos1
       assert @branch.to_s.include?("Branch")