diff --git a/presto-main/src/main/java/com/facebook/presto/operator/scalar/MapKeys.java b/presto-main/src/main/java/com/facebook/presto/operator/scalar/MapKeys.java
index ee6fd05dafc00..3697bbadb5dc4 100644
--- a/presto-main/src/main/java/com/facebook/presto/operator/scalar/MapKeys.java
+++ b/presto-main/src/main/java/com/facebook/presto/operator/scalar/MapKeys.java
@@ -69,7 +69,7 @@ public ScalarFunctionImplementation specialize(Map<String, Type> types, int arit
 
     public static Block getKeys(Type keyType, Block block)
     {
-        BlockBuilder blockBuilder = keyType.createBlockBuilder(new BlockBuilderStatus(), block.getSizeInBytes());
+        BlockBuilder blockBuilder = keyType.createBlockBuilder(new BlockBuilderStatus(), block.getPositionCount() / 2);
         for (int i = 0; i < block.getPositionCount(); i += 2) {
             keyType.appendTo(block, i, blockBuilder);
         }