diff --git a/lib/diaspora_federation/validators/rules/guid.rb b/lib/diaspora_federation/validators/rules/guid.rb
index 73077fa..821f0a5 100644
--- a/lib/diaspora_federation/validators/rules/guid.rb
+++ b/lib/diaspora_federation/validators/rules/guid.rb
@@ -7,6 +7,21 @@ module Validation
     # * Numbers: 0-9
     # * Special chars: '-', '_', '@', '.' and ':'
     class Guid
+      # This rule can have a +nilable+ param
+      # @return [Hash] params
+      attr_reader :params
+
+      # create a new rule for guid validation
+      # @param [Hash] params
+      # @option params [Boolean] :nilable guid allowed to be nil
+      def initialize(params={})
+        if params.include?(:nilable) && !params[:nilable].is_a?(TrueClass) && !params[:nilable].is_a?(FalseClass)
+          raise ArgumentError, ":nilable needs to be a boolean"
+        end
+
+        @params = params
+      end
+
       # The error key for this rule
       # @return [Symbol] error key
       def error_key
@@ -15,13 +30,7 @@ module Validation
 
       # Determines if value is a valid +GUID+
       def valid_value?(value)
-        value.is_a?(String) && value.downcase =~ /\A[0-9a-z\-_@.:]{16,}\z/
-      end
-
-      # This rule has no params
-      # @return [Hash] params
-      def params
-        {}
+        params[:nilable] && value.nil? || value.is_a?(String) && value.downcase =~ /\A[0-9a-z\-_@.:]{16,}\z/
       end
     end
   end
diff --git a/spec/lib/diaspora_federation/validators/rules/guid_spec.rb b/spec/lib/diaspora_federation/validators/rules/guid_spec.rb
index ea63bef..e2d48f8 100644
--- a/spec/lib/diaspora_federation/validators/rules/guid_spec.rb
+++ b/spec/lib/diaspora_federation/validators/rules/guid_spec.rb
@@ -1,9 +1,25 @@
 describe Validation::Rule::Guid do
-  it "will not accept parameters" do
+  it "allows a nilable parameter" do
     validator = Validation::Validator.new({})
     expect {
-      validator.rule(:guid, guid: {param: true})
-    }.to raise_error ArgumentError
+      validator.rule(:guid, guid: {nilable: false})
+    }.not_to raise_error
+  end
+
+  it "doesn't require a nilable parameter" do
+    validator = Validation::Validator.new({})
+    expect {
+      validator.rule(:guid, :guid)
+    }.not_to raise_error
+  end
+
+  it "requires a boolean as parameter" do
+    validator = Validation::Validator.new({})
+    [nil, "", 42, 5.5].each do |val|
+      expect {
+        validator.rule(:guid, guid: {nilable: val})
+      }.to raise_error ArgumentError, ":nilable needs to be a boolean"
+    end
   end
 
   it "has an error key" do
@@ -28,6 +44,14 @@ describe Validation::Rule::Guid do
       expect(validator.errors).to be_empty
     end
 
+    it "validates a nil guid if nilable is true" do
+      validator = Validation::Validator.new(OpenStruct.new(guid: nil))
+      validator.rule(:guid, guid: {nilable: true})
+
+      expect(validator).to be_valid
+      expect(validator.errors).to be_empty
+    end
+
     it "fails if the string is too short" do
       validator = Validation::Validator.new(OpenStruct.new(guid: "012345"))
       validator.rule(:guid, :guid)
@@ -44,6 +68,14 @@ describe Validation::Rule::Guid do
       expect(validator.errors).to include(:guid)
     end
 
+    it "fails if the string contains invalid chars if nilable is true" do
+      validator = Validation::Validator.new(OpenStruct.new(guid: "ghijklmnopqrstuvwxyz++"))
+      validator.rule(:guid, guid: {nilable: true})
+
+      expect(validator).not_to be_valid
+      expect(validator.errors).to include(:guid)
+    end
+
     it "fails if the string is empty" do
       [nil, ""].each do |val|
         validator = Validation::Validator.new(OpenStruct.new(guid: val))