diff --git a/src/main/java/org/springframework/data/elasticsearch/Document.java b/src/main/java/org/springframework/data/elasticsearch/Document.java
index 703e40c25..5e68afebd 100644
--- a/src/main/java/org/springframework/data/elasticsearch/Document.java
+++ b/src/main/java/org/springframework/data/elasticsearch/Document.java
@@ -37,6 +37,7 @@ import org.springframework.util.Assert;
  * {@link UnsupportedOperationException} when calling modifying methods.
  *
  * @author Mark Paluch
+ * @author Peter-Josef Meisch
  * @since 4.0
  */
 public interface Document extends Map<String, Object> {
@@ -53,7 +54,7 @@ public interface Document extends Map<String, Object> {
 	/**
 	 * Create a {@link Document} from a {@link Map} containing key-value pairs and sub-documents.
 	 *
-	 * @param map source map containing key-value pairs and sub-documents.
+	 * @param map source map containing key-value pairs and sub-documents. must not be {@literal null}.
 	 * @return a new {@link Document}.
 	 */
 	static Document from(Map<String, Object> map) {
@@ -74,6 +75,9 @@ public interface Document extends Map<String, Object> {
 	 * @return the parsed {@link Document}.
 	 */
 	static Document parse(String json) {
+
+		Assert.notNull(json, "JSON must not be null");
+
 		try {
 			return new MapDocument(MapDocument.OBJECT_MAPPER.readerFor(Map.class).readValue(json));
 		} catch (IOException e) {
@@ -84,11 +88,14 @@ public interface Document extends Map<String, Object> {
 	/**
 	 * {@link #put(Object, Object)} the {@code key}/{@code value} tuple and return {@code this} {@link Document}.
 	 *
-	 * @param key key with which the specified value is to be associated.
+	 * @param key key with which the specified value is to be associated. must not be {@literal null}.
 	 * @param value value to be associated with the specified key.
 	 * @return {@code this} {@link Document}.
 	 */
 	default Document append(String key, Object value) {
+
+		Assert.notNull(key, "Key must not be null");
+
 		put(key, value);
 		return this;
 	}
@@ -369,12 +376,15 @@ public interface Document extends Map<String, Object> {
 	 * <p>
 	 * Any exception thrown by the function will be propagated to the caller.
 	 *
-	 * @param transformer functional interface to a apply
+	 * @param transformer functional interface to a apply. must not be {@literal null}.
 	 * @param <R> class of the result
 	 * @return the result of applying the function to this string
 	 * @see java.util.function.Function
 	 */
 	default <R> R transform(Function<? super Document, ? extends R> transformer) {
+
+		Assert.notNull(transformer, "transformer must not be null");
+
 		return transformer.apply(this);
 	}
 
diff --git a/src/main/java/org/springframework/data/elasticsearch/core/DocumentAdapters.java b/src/main/java/org/springframework/data/elasticsearch/core/DocumentAdapters.java
index 9315da5c2..be965a8a6 100644
--- a/src/main/java/org/springframework/data/elasticsearch/core/DocumentAdapters.java
+++ b/src/main/java/org/springframework/data/elasticsearch/core/DocumentAdapters.java
@@ -44,8 +44,9 @@ import com.fasterxml.jackson.core.JsonFactory;
 import com.fasterxml.jackson.core.JsonGenerator;
 
 /**
- * Utility class to adapt {@link org.elasticsearch.common.document.DocumentField} to
- * {@link org.springframework.data.elasticsearch.Document}.
+ * Utility class to adapt {@link org.elasticsearch.action.get.GetResponse},
+ * {@link org.elasticsearch.index.get.GetResult}, {@link org.elasticsearch.search.SearchHit},
+ * {@link org.elasticsearch.common.document.DocumentField} to {@link org.springframework.data.elasticsearch.Document}.
  *
  * @author Mark Paluch
  * @since 4.0
diff --git a/src/test/java/org/springframework/data/elasticsearch/core/DocumentAdaptersUnitTests.java b/src/test/java/org/springframework/data/elasticsearch/core/DocumentAdaptersUnitTests.java
index 77804f211..5417e88cc 100644
--- a/src/test/java/org/springframework/data/elasticsearch/core/DocumentAdaptersUnitTests.java
+++ b/src/test/java/org/springframework/data/elasticsearch/core/DocumentAdaptersUnitTests.java
@@ -29,7 +29,6 @@ import org.elasticsearch.common.text.Text;
 import org.elasticsearch.index.get.GetResult;
 import org.elasticsearch.search.SearchHit;
 import org.junit.Test;
-
 import org.springframework.data.elasticsearch.Document;
 import org.springframework.data.elasticsearch.SearchDocument;
 
@@ -37,6 +36,7 @@ import org.springframework.data.elasticsearch.SearchDocument;
  * Unit tests for {@link DocumentAdapters}.
  *
  * @author Mark Paluch
+ * @author Peter-Josef Meisch
  */
 public class DocumentAdaptersUnitTests {
 
@@ -46,7 +46,7 @@ public class DocumentAdaptersUnitTests {
 		Map<String, DocumentField> fields = Collections.singletonMap("field",
 				new DocumentField("field", Arrays.asList("value")));
 
-		GetResult getResult = new GetResult("index", "type", "my-id", 1, 0, 42, true, null, fields);
+		GetResult getResult = new GetResult("index", "type", "my-id", 1, 1, 42, true, null, fields, null);
 		GetResponse response = new GetResponse(getResult);
 
 		Document document = DocumentAdapters.from(response);
@@ -63,7 +63,7 @@ public class DocumentAdaptersUnitTests {
 
 		BytesArray source = new BytesArray("{\"field\":\"value\"}");
 
-		GetResult getResult = new GetResult("index", "type", "my-id", 1, 0, 42, true, source, Collections.emptyMap());
+		GetResult getResult = new GetResult("index", "type", "my-id", 1, 1, 42, true, source, Collections.emptyMap(), null);
 		GetResponse response = new GetResponse(getResult);
 
 		Document document = DocumentAdapters.from(response);