diff --git a/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/GroupOperation.java b/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/GroupOperation.java
index 1b03937d0..f4cff0f82 100644
--- a/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/GroupOperation.java
+++ b/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/GroupOperation.java
@@ -160,10 +160,14 @@ public class GroupOperation implements FieldsExposingAggregationOperation {
 	 * Generates an {@link GroupOperationBuilder} for an {@code $sum}-expression for the given
 	 * {@link AggregationExpression}.
 	 *
-	 * @param expr
-	 * @return
+	 * @param expr must not be {@literal null}.
+	 * @return new instance of {@link GroupOperationBuilder}. Never {@literal null}.
+	 * @throws IllegalArgumentException when {@code expr} is {@literal null}.
+	 * @since 1.10.8
 	 */
 	public GroupOperationBuilder sum(AggregationExpression expr) {
+
+		Assert.notNull(expr, "Expr must not be null!");
 		return newBuilder(GroupOps.SUM, null, expr);
 	}
 
diff --git a/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/AggregationTests.java b/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/AggregationTests.java
index 0b2c4b8cb..11b39e7dd 100644
--- a/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/AggregationTests.java
+++ b/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/AggregationTests.java
@@ -835,11 +835,11 @@ public class AggregationTests {
 		assertThat(result.getMappedResults(), hasSize(2));
 
 		Document meh = result.getMappedResults().get(0);
-		assertThat((String) meh.get("_id"), is(equalTo("meh")));
+		assertThat(meh.get("_id"), is(equalTo("meh")));
 		assertThat(((Number) meh.get("score")).longValue(), is(equalTo(2L)));
 
 		Document good = result.getMappedResults().get(1);
-		assertThat((String) good.get("_id"), is(equalTo("good")));
+		assertThat(good.get("_id"), is(equalTo("good")));
 		assertThat(((Number) good.get("score")).longValue(), is(equalTo(18000L)));
 	}
 
diff --git a/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/GroupOperationUnitTests.java b/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/GroupOperationUnitTests.java
index e6684db0b..e8da2f591 100644
--- a/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/GroupOperationUnitTests.java
+++ b/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/GroupOperationUnitTests.java
@@ -25,6 +25,7 @@ import java.util.Arrays;
 import org.bson.Document;
 import org.junit.Test;
 import org.springframework.data.mongodb.core.DocumentTestUtils;
+import org.springframework.data.mongodb.core.query.Criteria;
 
 /**
  * Unit tests for {@link GroupOperation}.
@@ -216,6 +217,29 @@ public class GroupOperationUnitTests {
 		assertThat(push, is(new Document("$stdDevPop", "$field")));
 	}
 
+	@Test // DATAMONGO-1784
+	public void shouldRenderSumWithExpressionInGroup() {
+
+		GroupOperation groupOperation = Aggregation //
+				.group("username") //
+				.sum(ConditionalOperators //
+						.when(Criteria.where("foo").is("bar")) //
+						.then(1) //
+						.otherwise(-1)) //
+				.as("foobar");
+
+		Document groupClause = extractDocumentFromGroupOperation(groupOperation);
+		Document foobar = DocumentTestUtils.getAsDocument(groupClause, "foobar");
+
+		assertThat(foobar.get("$sum"), is(new Document("$cond",
+				new Document("if", new Document("$eq", Arrays.asList("$foo", "bar"))).append("then", 1).append("else", -1))));
+	}
+
+	@Test(expected = IllegalArgumentException.class) // DATAMONGO-1784
+	public void sumWithNullExpressionShouldThrowException() {
+		Aggregation.group("username").sum((AggregationExpression) null);
+	}
+
 	private Document extractDocumentFromGroupOperation(GroupOperation groupOperation) {
 		Document document = groupOperation.toDocument(Aggregation.DEFAULT_CONTEXT);
 		Document groupClause = DocumentTestUtils.getAsDocument(document, "$group");