diff --git a/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/repository/query/AggregationUtils.java b/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/repository/query/AggregationUtils.java
index 2333917f3..c06a7a691 100644
--- a/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/repository/query/AggregationUtils.java
+++ b/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/repository/query/AggregationUtils.java
@@ -196,9 +196,9 @@ class AggregationUtils {
 	 * @throws IllegalArgumentException when none of the above rules is met.
 	 */
 	@Nullable
-	static <T> T extractSimpleTypeResult(Document source, Class<T> targetType, MongoConverter converter) {
+	static <T> T extractSimpleTypeResult(@Nullable Document source, Class<T> targetType, MongoConverter converter) {
 
-		if (source.isEmpty()) {
+		if (source == null || source.isEmpty()) {
 			return null;
 		}
 
diff --git a/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/repository/query/StringBasedAggregationUnitTests.java b/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/repository/query/StringBasedAggregationUnitTests.java
index cc0890785..7ff8825c0 100644
--- a/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/repository/query/StringBasedAggregationUnitTests.java
+++ b/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/repository/query/StringBasedAggregationUnitTests.java
@@ -156,6 +156,14 @@ public class StringBasedAggregationUnitTests {
 		assertThat(executeAggregation("returnCollection").result).isEqualTo(expected);
 	}
 
+	@Test // GH-3623
+	public void returnNullWhenSingleResultIsNotPresent() {
+
+		when(aggregationResults.getMappedResults()).thenReturn(Collections.emptyList());
+
+		assertThat(executeAggregation("simpleReturnType").result).isNull();
+	}
+
 	@Test // DATAMONGO-2153
 	public void returnRawResultType() {
 		assertThat(executeAggregation("returnRawResultType").result).isEqualTo(aggregationResults);
@@ -299,6 +307,9 @@ public class StringBasedAggregationUnitTests {
 
 		@Aggregation(RAW_GROUP_BY_LASTNAME_STRING)
 		Page<Person> invalidPageReturnType(Pageable page);
+
+		@Aggregation(RAW_GROUP_BY_LASTNAME_STRING)
+		String simpleReturnType();
 	}
 
 	static class PersonAggregate {