diff --git a/.travis.yml b/.travis.yml
index 8ba1558..fe3c026 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,8 @@
 language: php
 php:
   - '7.1'
+  - '7.2'
+  - '7.3'
 
 before_script:
   - composer install
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 86ca5ab..9185eeb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased]
 
+## [2.1.0] - 2019-02-25
+### Fixed
+- Relationship without data property (#92)
+
 ## [2.0.1] - 2018-12-31
 ### Changed
 - Downgraded min required php version to 7.1
@@ -14,5 +18,6 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ### Added
 - v2 initial release
 
-[Unreleased]: https://github.com/json-api-php/json-api/compare/2.0.1...HEAD
+[Unreleased]: https://github.com/json-api-php/json-api/compare/2.1.0...HEAD
+[2.1.0]: https://github.com/json-api-php/json-api/compare/2.0.1...2.1.0
 [2.0.1]: https://github.com/json-api-php/json-api/compare/2.0.0...2.0.1
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index 48bfa68..884a29a 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -3,8 +3,7 @@
         bootstrap="vendor/autoload.php"
         stopOnFailure="false"
         verbose="true"
-        colors="true"
-        syntaxCheck="true">
+        colors="true">
     <testsuites>
         <testsuite name="Main">
             <directory>./test</directory>
diff --git a/src/EmptyRelationship.php b/src/EmptyRelationship.php
new file mode 100644
index 0000000..1482792
--- /dev/null
+++ b/src/EmptyRelationship.php
@@ -0,0 +1,31 @@
+<?php declare(strict_types=1);
+
+namespace JsonApiPhp\JsonApi;
+
+use JsonApiPhp\JsonApi\Internal\RelationshipMember;
+use JsonApiPhp\JsonApi\Internal\ResourceField;
+use JsonApiPhp\JsonApi\Internal\ResourceFieldTrait;
+
+/**
+ * A relationship with no data
+ */
+class EmptyRelationship implements ResourceField
+{
+    use ResourceFieldTrait;
+
+    private $obj;
+
+    public function __construct(string $name, RelationshipMember $member, RelationshipMember ...$members)
+    {
+        $this->name = $name;
+        $this->obj = combine($member, ...$members);
+    }
+
+    /**
+     * @param object $o
+     */
+    public function attachTo($o): void
+    {
+        child($o, 'relationships')->{$this->name} = $this->obj;
+    }
+}
diff --git a/src/Error/SourceParameter.php b/src/Error/SourceParameter.php
index 05b711a..f3bc37a 100644
--- a/src/Error/SourceParameter.php
+++ b/src/Error/SourceParameter.php
@@ -2,8 +2,8 @@
 
 namespace JsonApiPhp\JsonApi\Error;
 
-use JsonApiPhp\JsonApi\Internal\ErrorMember;
 use function JsonApiPhp\JsonApi\child;
+use JsonApiPhp\JsonApi\Internal\ErrorMember;
 
 final class SourceParameter implements ErrorMember
 {
diff --git a/src/Error/SourcePointer.php b/src/Error/SourcePointer.php
index 84815db..0af0b5b 100644
--- a/src/Error/SourcePointer.php
+++ b/src/Error/SourcePointer.php
@@ -2,8 +2,8 @@
 
 namespace JsonApiPhp\JsonApi\Error;
 
-use JsonApiPhp\JsonApi\Internal\ErrorMember;
 use function JsonApiPhp\JsonApi\child;
+use JsonApiPhp\JsonApi\Internal\ErrorMember;
 
 final class SourcePointer implements ErrorMember
 {
diff --git a/src/Internal/RelationshipMember.php b/src/Internal/RelationshipMember.php
new file mode 100644
index 0000000..3746dee
--- /dev/null
+++ b/src/Internal/RelationshipMember.php
@@ -0,0 +1,10 @@
+<?php declare(strict_types=1);
+
+namespace JsonApiPhp\JsonApi\Internal;
+
+/**
+ * @internal
+ */
+interface RelationshipMember extends ToOneMember, ToManyMember
+{
+}
diff --git a/src/Internal/ToManyMember.php b/src/Internal/ToManyMember.php
index 52dd2f7..d812f11 100644
--- a/src/Internal/ToManyMember.php
+++ b/src/Internal/ToManyMember.php
@@ -2,6 +2,9 @@
 
 namespace JsonApiPhp\JsonApi\Internal;
 
-interface ToManyMember extends ToOneMember
+/**
+ * @internal
+ */
+interface ToManyMember extends Attachable
 {
 }
diff --git a/src/Link/AboutLink.php b/src/Link/AboutLink.php
index 054129e..6035330 100644
--- a/src/Link/AboutLink.php
+++ b/src/Link/AboutLink.php
@@ -2,9 +2,9 @@
 
 namespace JsonApiPhp\JsonApi\Link;
 
+use function JsonApiPhp\JsonApi\child;
 use JsonApiPhp\JsonApi\Internal\ErrorMember;
 use JsonApiPhp\JsonApi\Internal\LinkTrait;
-use function JsonApiPhp\JsonApi\child;
 
 final class AboutLink implements ErrorMember
 {
diff --git a/src/Link/FirstLink.php b/src/Link/FirstLink.php
index e569e86..755a12b 100644
--- a/src/Link/FirstLink.php
+++ b/src/Link/FirstLink.php
@@ -2,9 +2,9 @@
 
 namespace JsonApiPhp\JsonApi\Link;
 
+use function JsonApiPhp\JsonApi\child;
 use JsonApiPhp\JsonApi\Internal\LinkTrait;
 use JsonApiPhp\JsonApi\Internal\PaginationLink;
-use function JsonApiPhp\JsonApi\child;
 
 final class FirstLink implements PaginationLink
 {
diff --git a/src/Link/LastLink.php b/src/Link/LastLink.php
index 36a91e8..c11b2e0 100644
--- a/src/Link/LastLink.php
+++ b/src/Link/LastLink.php
@@ -2,9 +2,9 @@
 
 namespace JsonApiPhp\JsonApi\Link;
 
+use function JsonApiPhp\JsonApi\child;
 use JsonApiPhp\JsonApi\Internal\LinkTrait;
 use JsonApiPhp\JsonApi\Internal\PaginationLink;
-use function JsonApiPhp\JsonApi\child;
 
 final class LastLink implements PaginationLink
 {
diff --git a/src/Link/NextLink.php b/src/Link/NextLink.php
index 1b2caa1..39f5641 100644
--- a/src/Link/NextLink.php
+++ b/src/Link/NextLink.php
@@ -2,9 +2,9 @@
 
 namespace JsonApiPhp\JsonApi\Link;
 
+use function JsonApiPhp\JsonApi\child;
 use JsonApiPhp\JsonApi\Internal\LinkTrait;
 use JsonApiPhp\JsonApi\Internal\PaginationLink;
-use function JsonApiPhp\JsonApi\child;
 
 final class NextLink implements PaginationLink
 {
diff --git a/src/Link/PrevLink.php b/src/Link/PrevLink.php
index a02c69b..f38d94b 100644
--- a/src/Link/PrevLink.php
+++ b/src/Link/PrevLink.php
@@ -2,9 +2,9 @@
 
 namespace JsonApiPhp\JsonApi\Link;
 
+use function JsonApiPhp\JsonApi\child;
 use JsonApiPhp\JsonApi\Internal\LinkTrait;
 use JsonApiPhp\JsonApi\Internal\PaginationLink;
-use function JsonApiPhp\JsonApi\child;
 
 final class PrevLink implements PaginationLink
 {
diff --git a/src/Link/RelatedLink.php b/src/Link/RelatedLink.php
index 70ddc34..7434423 100644
--- a/src/Link/RelatedLink.php
+++ b/src/Link/RelatedLink.php
@@ -2,11 +2,11 @@
 
 namespace JsonApiPhp\JsonApi\Link;
 
-use JsonApiPhp\JsonApi\Internal\LinkTrait;
-use JsonApiPhp\JsonApi\Internal\ToOneMember;
 use function JsonApiPhp\JsonApi\child;
+use JsonApiPhp\JsonApi\Internal\LinkTrait;
+use JsonApiPhp\JsonApi\Internal\RelationshipMember;
 
-final class RelatedLink implements ToOneMember
+final class RelatedLink implements RelationshipMember
 {
     use LinkTrait;
 
diff --git a/src/Link/SelfLink.php b/src/Link/SelfLink.php
index f886435..5a9a31f 100644
--- a/src/Link/SelfLink.php
+++ b/src/Link/SelfLink.php
@@ -2,13 +2,13 @@
 
 namespace JsonApiPhp\JsonApi\Link;
 
+use function JsonApiPhp\JsonApi\child;
 use JsonApiPhp\JsonApi\Internal\DataDocumentMember;
 use JsonApiPhp\JsonApi\Internal\LinkTrait;
+use JsonApiPhp\JsonApi\Internal\RelationshipMember;
 use JsonApiPhp\JsonApi\Internal\ResourceMember;
-use JsonApiPhp\JsonApi\Internal\ToOneMember;
-use function JsonApiPhp\JsonApi\child;
 
-final class SelfLink implements DataDocumentMember, ResourceMember, ToOneMember
+final class SelfLink implements DataDocumentMember, ResourceMember, RelationshipMember
 {
     use LinkTrait;
 
diff --git a/src/Meta.php b/src/Meta.php
index a267bdb..5b9a2c3 100644
--- a/src/Meta.php
+++ b/src/Meta.php
@@ -6,10 +6,10 @@
 use JsonApiPhp\JsonApi\Internal\ErrorDocumentMember;
 use JsonApiPhp\JsonApi\Internal\ErrorMember;
 use JsonApiPhp\JsonApi\Internal\MetaDocumentMember;
+use JsonApiPhp\JsonApi\Internal\RelationshipMember;
 use JsonApiPhp\JsonApi\Internal\ResourceMember;
-use JsonApiPhp\JsonApi\Internal\ToOneMember;
 
-final class Meta implements ErrorMember, ErrorDocumentMember, MetaDocumentMember, DataDocumentMember, ResourceMember, ToOneMember
+final class Meta implements ErrorMember, ErrorDocumentMember, MetaDocumentMember, DataDocumentMember, ResourceMember, RelationshipMember
 {
     /**
      * @var string
diff --git a/src/ToMany.php b/src/ToMany.php
index e4227eb..0811006 100644
--- a/src/ToMany.php
+++ b/src/ToMany.php
@@ -5,13 +5,13 @@
 use JsonApiPhp\JsonApi\Internal\Identifier;
 use JsonApiPhp\JsonApi\Internal\ResourceField;
 use JsonApiPhp\JsonApi\Internal\ResourceFieldTrait;
-use JsonApiPhp\JsonApi\Internal\ToOneMember;
+use JsonApiPhp\JsonApi\Internal\ToManyMember;
 
 final class ToMany implements Identifier, ResourceField
 {
     use ResourceFieldTrait;
     /**
-     * @var ToOneMember[]
+     * @var ToManyMember[]
      */
     private $members;
     /**
@@ -19,7 +19,7 @@ final class ToMany implements Identifier, ResourceField
      */
     private $collection;
 
-    public function __construct(string $name, ResourceIdentifierCollection $collection, ToOneMember ...$members)
+    public function __construct(string $name, ResourceIdentifierCollection $collection, ToManyMember ...$members)
     {
         $this->validateFieldName($name);
         $this->name = $name;
diff --git a/test/CompoundDocumentTest.php b/test/CompoundDocumentTest.php
index 3d62ff9..7441200 100644
--- a/test/CompoundDocumentTest.php
+++ b/test/CompoundDocumentTest.php
@@ -253,11 +253,11 @@ public function testIncludedResourceMayBeIdentifiedByAnotherLinkedResource()
 
     /**
      * A compound document MUST NOT include more than one resource object for each type and id pair.
-     * @expectedException \LogicException
-     * @expectedExceptionMessage Resource apples:1 is already included
      */
     public function testCanNotBeManyIncludedResourcesWithEqualIdentifiers()
     {
+        $this->expectException('LogicException');
+        $this->expectExceptionMessage('Resource apples:1 is already included');
         $apple = new ResourceObject('apples', '1');
         new CompoundDocument($apple->identifier(), new Included($apple, $apple));
     }
diff --git a/test/ResourceObjectTest.php b/test/ResourceObjectTest.php
index 37e845f..3541c8d 100644
--- a/test/ResourceObjectTest.php
+++ b/test/ResourceObjectTest.php
@@ -4,6 +4,7 @@
 
 use JsonApiPhp\JsonApi\Attribute;
 use JsonApiPhp\JsonApi\DataDocument;
+use JsonApiPhp\JsonApi\EmptyRelationship;
 use JsonApiPhp\JsonApi\Link\RelatedLink;
 use JsonApiPhp\JsonApi\Link\SelfLink;
 use JsonApiPhp\JsonApi\Meta;
@@ -152,6 +153,63 @@ public function testRelationshipWithEmptyMultiIdLinkage()
         );
     }
 
+    public function testRelationshipWithNoData()
+    {
+        $this->assertEncodesTo(
+            '
+            {
+                "data": {
+                    "type": "basket",
+                    "id": "1",
+                    "relationships": {
+                        "empty": {
+                            "links": {
+                                "related": "/foo"
+                            }
+                        }
+                    }
+                }
+            }
+            ',
+            new DataDocument(
+                new ResourceObject(
+                    'basket',
+                    '1',
+                    new EmptyRelationship('empty', new RelatedLink('/foo'))
+                )
+            )
+        );
+
+        $this->assertEncodesTo(
+            '
+            {
+                "data": {
+                    "type": "basket",
+                    "id": "1",
+                    "relationships": {
+                        "empty": {
+                            "links": {
+                                "related": "/foo",
+                                "self": "/bar"
+                            },
+                            "meta": {
+                                "foo": "bar"
+                            }
+                        }
+                    }
+                }
+            }
+            ',
+            new DataDocument(
+                new ResourceObject(
+                    'basket',
+                    '1',
+                    new EmptyRelationship('empty', new RelatedLink('/foo'), new SelfLink('/bar'), new Meta('foo', 'bar'))
+                )
+            )
+        );
+    }
+
     public function testCanNotCreateIdAttribute()
     {
         $this->expectException(\DomainException::class);

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