diff --git a/presto-spi/src/main/java/com/facebook/presto/spi/statistics/ColumnStatistics.java b/presto-spi/src/main/java/com/facebook/presto/spi/statistics/ColumnStatistics.java
index 135abdfb3fe4d..14a8bccd1eedd 100644
--- a/presto-spi/src/main/java/com/facebook/presto/spi/statistics/ColumnStatistics.java
+++ b/presto-spi/src/main/java/com/facebook/presto/spi/statistics/ColumnStatistics.java
@@ -114,6 +114,12 @@ public static Builder builder()
         return new Builder();
     }
 
+    /**
+     * If one of the estimates below is unspecified, the default "unknown" estimate value
+     * (represented by floating point NaN) may cause the resulting symbol statistics
+     * to be "unknown" as well.
+     * @see SymbolStatsEstimate
+     */
     public static final class Builder
     {
         private Estimate nullsFraction = Estimate.unknown();