diff --git a/core/src/test/scala/org/apache/spark/sql/test/generator/ColumnValueGenerator.scala b/core/src/test/scala/org/apache/spark/sql/test/generator/ColumnValueGenerator.scala
index 9cf36ecbb7..03cbf0d69f 100644
--- a/core/src/test/scala/org/apache/spark/sql/test/generator/ColumnValueGenerator.scala
+++ b/core/src/test/scala/org/apache/spark/sql/test/generator/ColumnValueGenerator.scala
@@ -236,7 +236,7 @@ case class ColumnValueGenerator(dataType: ReflectedDataType,
       generatedRandomValues = if (generateUnique) {
         assert(n <= rangeSize, "random generator cannot generate unique value less than available")
         val set: mutable.Set[Any] = mutable.HashSet.empty[Any]
-        set += specialBound.map(TestDataGenerator.hash)
+        set ++= specialBound.map(TestDataGenerator.hash)
         (0L until n - specialBound.size).map { _ =>
           randomUniqueValue(r, set)
         }.toList ++ specialBound
@@ -245,9 +245,16 @@ case class ColumnValueGenerator(dataType: ReflectedDataType,
           randomValue(r)
         }.toList ++ specialBound
       }
+
+      val expectedGeneratedRandomValuesLen = if (generateUnique) {
+        generatedRandomValues.toSet.size
+      } else {
+        generatedRandomValues.size
+      }
+
       assert(
-        generatedRandomValues.size >= n,
-        s"Generate values size=$generatedRandomValues less than n=$n"
+        expectedGeneratedRandomValuesLen >= n,
+        s"Generate values size=$generatedRandomValues less than n=$n on datatype $dataType"
       )
       curPos = 0
     }