diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkPlan.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkPlan.scala
index 9a338e51268f3..c6c45e7e098c8 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkPlan.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkPlan.scala
@@ -98,6 +98,8 @@ abstract class SparkPlan extends QueryPlan[SparkPlan] with Logging with Serializ
    * Concrete implementations of SparkPlan should override doExecute instead.
    */
   final def execute(): RDD[InternalRow] = {
+    assert(children.map(_.outputsUnsafeRows).distinct.length <= 1,
+      "Child operators should output rows in the same format")
     RDDOperationScope.withScope(sparkContext, nodeName, false, true) {
       doExecute()
     }
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/execution/RowFormatConvertersSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/execution/RowFormatConvertersSuite.scala
index 9dfd7c7fa975b..2b10f5654ec9d 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/execution/RowFormatConvertersSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/execution/RowFormatConvertersSuite.scala
@@ -28,23 +28,35 @@ class RowFormatConvertersSuite extends SparkFunSuite {
     case c: ConvertFromUnsafe => c
   }
 
+  private val outputsSafe = ExternalSort(Nil, false, PhysicalRDD(Seq.empty, null))
+  assert(!outputsSafe.outputsUnsafeRows)
+  private val outputsUnsafe = UnsafeExternalSort(Nil, false, PhysicalRDD(Seq.empty, null))
+  assert(outputsUnsafe.outputsUnsafeRows)
+
   test("planner should insert unsafe->safe conversions when required") {
-    val plan = Limit(10, UnsafeExternalSort(Nil, false, PhysicalRDD(Seq.empty, null)))
+    val plan = Limit(10, outputsUnsafe)
     val preparedPlan = TestSQLContext.prepareForExecution.execute(plan)
     assert(preparedPlan.children.head.isInstanceOf[ConvertFromUnsafe])
   }
 
   test("filter can process unsafe rows") {
-    val plan = Filter(IsNull(null), UnsafeExternalSort(Nil, false, PhysicalRDD(Seq.empty, null)))
-    assert(plan.child.outputsUnsafeRows)
+    val plan = Filter(IsNull(null), outputsUnsafe)
     val preparedPlan = TestSQLContext.prepareForExecution.execute(plan)
     assert(getConverters(preparedPlan).isEmpty)
+    assert(preparedPlan.outputsUnsafeRows)
   }
 
   test("filter can process safe rows") {
-    val plan = Filter(IsNull(null), ExternalSort(Nil, false, PhysicalRDD(Seq.empty, null)))
-    assert(!plan.child.outputsUnsafeRows)
+    val plan = Filter(IsNull(null), outputsSafe)
     val preparedPlan = TestSQLContext.prepareForExecution.execute(plan)
     assert(getConverters(preparedPlan).isEmpty)
+    assert(!preparedPlan.outputsUnsafeRows)
+  }
+
+  test("execute() fails an assertion if inputs rows are of different formats") {
+    val e = intercept[AssertionError] {
+      Union(Seq(outputsSafe, outputsUnsafe)).execute()
+    }
+    assert(e.getMessage.contains("format"))
   }
 }