diff --git a/core/src/main/java/org/apache/calcite/runtime/SqlFunctions.java b/core/src/main/java/org/apache/calcite/runtime/SqlFunctions.java
index b0bd8dfe224a..59741c10bab1 100644
--- a/core/src/main/java/org/apache/calcite/runtime/SqlFunctions.java
+++ b/core/src/main/java/org/apache/calcite/runtime/SqlFunctions.java
@@ -321,7 +321,7 @@ public static ByteString fromHex(String hex) {
   }
 
   /** SQL UNHEX(varchar) function. */
-  public static ByteString unHex(String hex) {
+  public static @Nullable ByteString unHex(String hex) {
     try {
       return fromHexHelper(hex);
     } catch (DecoderException e) {