diff --git a/lib/diaspora_federation/schemas/federation_entities.json b/lib/diaspora_federation/schemas/federation_entities.json
index 1226694..98ec53a 100644
--- a/lib/diaspora_federation/schemas/federation_entities.json
+++ b/lib/diaspora_federation/schemas/federation_entities.json
@@ -17,8 +17,17 @@
 
   "definitions": {
     "signature": {
-      "type": "string",
-      "minLength": 30
+      "oneOf" : [
+        {
+          "type": "string",
+          "minLength": 30
+        },
+        {
+          "type": "string",
+          "maxLength": 0,
+          "description": "Allow empty string when no signature is provided"
+        }
+      ]
     },
 
     "guid": {
diff --git a/spec/lib/diaspora_federation/entities/comment_spec.rb b/spec/lib/diaspora_federation/entities/comment_spec.rb
index c59cfc9..e47a364 100644
--- a/spec/lib/diaspora_federation/entities/comment_spec.rb
+++ b/spec/lib/diaspora_federation/entities/comment_spec.rb
@@ -55,6 +55,8 @@ JSON
 
     it_behaves_like "a relayable Entity"
 
+    it_behaves_like "a relayable JSON entity"
+
     describe "#created_at" do
       it "has a created_at after parse" do
         entity = described_class.from_xml(Nokogiri::XML::Document.parse(xml).root)
diff --git a/spec/lib/diaspora_federation/entities/like_spec.rb b/spec/lib/diaspora_federation/entities/like_spec.rb
index 800f0bc..ae3534d 100644
--- a/spec/lib/diaspora_federation/entities/like_spec.rb
+++ b/spec/lib/diaspora_federation/entities/like_spec.rb
@@ -56,6 +56,8 @@ JSON
 
     it_behaves_like "a relayable Entity"
 
+    it_behaves_like "a relayable JSON entity"
+
     context "invalid XML" do
       it "raises a ValidationError if the parent_type is missing" do
         broken_xml = <<-XML
diff --git a/spec/lib/diaspora_federation/entities/participation_spec.rb b/spec/lib/diaspora_federation/entities/participation_spec.rb
index 2ceb84e..4f7a34c 100644
--- a/spec/lib/diaspora_federation/entities/participation_spec.rb
+++ b/spec/lib/diaspora_federation/entities/participation_spec.rb
@@ -53,6 +53,8 @@ JSON
 
     it_behaves_like "a relayable Entity"
 
+    it_behaves_like "a relayable JSON entity"
+
     describe "#sender_valid?" do
       let(:entity) { Entities::Participation.new(data) }
 
diff --git a/spec/lib/diaspora_federation/entities/poll_participation_spec.rb b/spec/lib/diaspora_federation/entities/poll_participation_spec.rb
index 8d64df5..e81c52e 100644
--- a/spec/lib/diaspora_federation/entities/poll_participation_spec.rb
+++ b/spec/lib/diaspora_federation/entities/poll_participation_spec.rb
@@ -51,5 +51,7 @@ JSON
     it_behaves_like "a JSON Entity"
 
     it_behaves_like "a relayable Entity"
+
+    it_behaves_like "a relayable JSON entity"
   end
 end
diff --git a/spec/support/shared_entity_specs.rb b/spec/support/shared_entity_specs.rb
index 9f5c9f5..b869fc0 100644
--- a/spec/support/shared_entity_specs.rb
+++ b/spec/support/shared_entity_specs.rb
@@ -188,3 +188,12 @@ shared_examples "a JSON Entity" do
     expect(described_class.from_json(JSON.parse(entity_json)).to_json.to_json).to eq(entity_json)
   end
 end
+
+shared_examples "a relayable JSON entity" do
+  it "matches JSON schema with empty string signatures" do
+    json = described_class.new(data).to_json
+    json[:entity_data][:author_signature] = ""
+    json[:entity_data][:parent_author_signature] = ""
+    expect(json.to_json).to match_json_schema(:entity_schema)
+  end
+end