diff --git a/lib/gitlab/ci/pipeline/expression/null.rb b/lib/gitlab/ci/pipeline/expression/null.rb
index a70cdd5fc73..31dcba05b48 100644
--- a/lib/gitlab/ci/pipeline/expression/null.rb
+++ b/lib/gitlab/ci/pipeline/expression/null.rb
@@ -6,7 +6,7 @@ module Gitlab
           PATTERN = /null/.freeze
           TYPE = :value
 
-          def initialize(value)
+          def initialize(value = nil)
             @value = value
           end
 
diff --git a/lib/gitlab/ci/pipeline/expression/parser.rb b/lib/gitlab/ci/pipeline/expression/parser.rb
index 6eb77351dcf..350c9e543ac 100644
--- a/lib/gitlab/ci/pipeline/expression/parser.rb
+++ b/lib/gitlab/ci/pipeline/expression/parser.rb
@@ -4,9 +4,7 @@ module Gitlab
       module Expression
         class Parser
           def initialize(tokens)
-            # raise ArgumentError unless tokens.enumerator?
-
-            @tokens = tokens
+            @tokens = tokens.to_enum
             @nodes = []
           end
 
@@ -14,10 +12,7 @@ module Gitlab
             while token = @tokens.next
               case token.type
               when :operator
-                lookbehind = @nodes.last
-                lookahead = Parser.new(@tokens).tree
-
-                token.build(lookbehind, lookahead).tap do |node|
+                token.build(@nodes.last, tree).tap do |node|
                   @nodes.push(node)
                 end
               when :value
@@ -27,7 +22,11 @@ module Gitlab
               end
             end
           rescue StopIteration
-            @nodes.last
+            @nodes.last || Expression::Null.new
+          end
+
+          def self.seed(statement)
+            new(Expression::Lexer.new(statement).tokens)
           end
         end
       end
diff --git a/lib/gitlab/ci/pipeline/expression/statement.rb b/lib/gitlab/ci/pipeline/expression/statement.rb
index 07b84c89899..cb76bc00764 100644
--- a/lib/gitlab/ci/pipeline/expression/statement.rb
+++ b/lib/gitlab/ci/pipeline/expression/statement.rb
@@ -29,7 +29,7 @@ module Gitlab
               raise StatementError, 'Unknown pipeline expression!'
             end
 
-            Expression::Parser.new(@lexer.tokens.to_enum).tree
+            Expression::Parser.new(@lexer.tokens).tree
           end
 
           def evaluate
diff --git a/spec/lib/gitlab/ci/pipeline/expression/lexer_spec.rb b/spec/lib/gitlab/ci/pipeline/expression/lexer_spec.rb
index 464b74c54d8..0d70ca6c906 100644
--- a/spec/lib/gitlab/ci/pipeline/expression/lexer_spec.rb
+++ b/spec/lib/gitlab/ci/pipeline/expression/lexer_spec.rb
@@ -67,5 +67,4 @@ describe Gitlab::Ci::Pipeline::Expression::Lexer do
       expect(lexer.lexemes).to eq %w[variable string]
     end
   end
-
 end
diff --git a/spec/lib/gitlab/ci/pipeline/expression/parser_spec.rb b/spec/lib/gitlab/ci/pipeline/expression/parser_spec.rb
index c70bcc8438c..63c3393f07a 100644
--- a/spec/lib/gitlab/ci/pipeline/expression/parser_spec.rb
+++ b/spec/lib/gitlab/ci/pipeline/expression/parser_spec.rb
@@ -2,22 +2,25 @@ require 'spec_helper'
 
 describe Gitlab::Ci::Pipeline::Expression::Parser do
   describe '#tree' do
-    context 'when using an operator' do
+    context 'when using operators' do
       it 'returns a reverse descent parse tree' do
-        expect(described_class.new(tokens('$VAR == "123"')).tree)
+        expect(described_class.seed('$VAR1 == "123" == $VAR2').tree)
           .to be_a Gitlab::Ci::Pipeline::Expression::Equals
       end
     end
 
     context 'when using a single token' do
       it 'returns a single token instance' do
-        expect(described_class.new(tokens('$VAR')).tree)
+        expect(described_class.seed('$VAR').tree)
           .to be_a Gitlab::Ci::Pipeline::Expression::Variable
       end
     end
-  end
 
-  def tokens(statement)
-    Gitlab::Ci::Pipeline::Expression::Lexer.new(statement).tokens.to_enum
+    context 'when expression is empty' do
+      it 'returns a null token' do
+        expect(described_class.seed('').tree)
+          .to be_a Gitlab::Ci::Pipeline::Expression::Null
+      end
+    end
   end
 end