diff --git a/velox/docs/functions/spark/string.rst b/velox/docs/functions/spark/string.rst
index 64083dacb652..197bf4e5dd1d 100644
--- a/velox/docs/functions/spark/string.rst
+++ b/velox/docs/functions/spark/string.rst
@@ -121,7 +121,7 @@ Unless specified otherwise, all functions return NULL if at least one of the arg
 
         SELECT ltrim('ps', 'spark'); -- "ark"
 
-.. spark:function:: mask(string, upperChar, lowerChar, digitChar, otherChar) -> string
+.. spark:function:: mask(string[, upperChar, lowerChar, digitChar, otherChar]) -> string
 
     Returns a masked version of the input ``string``.
     ``string``: string value to mask.
diff --git a/velox/functions/sparksql/Mask.cpp b/velox/functions/sparksql/Mask.cpp
index 249f27d94ec3..a3bc1775851d 100644
--- a/velox/functions/sparksql/Mask.cpp
+++ b/velox/functions/sparksql/Mask.cpp
@@ -24,13 +24,7 @@
 namespace facebook::velox::functions::sparksql {
 namespace {
 class MaskFunction final : public exec::VectorFunction {
-  static constexpr std::string_view maskedUpperCase{"X"};
-  static constexpr std::string_view maskedLowerCase{"x"};
-  static constexpr std::string_view maskedDigit{"n"};
-
  public:
-  MaskFunction() {}
-
   void apply(
       const SelectivityVector& rows,
       std::vector<VectorPtr>& args,
@@ -179,6 +173,11 @@ class MaskFunction final : public exec::VectorFunction {
     }
     result.resize(inputSize);
   }
+
+ private:
+  static constexpr std::string_view maskedUpperCase{"X"};
+  static constexpr std::string_view maskedLowerCase{"x"};
+  static constexpr std::string_view maskedDigit{"n"};
 };
 
 std::shared_ptr<exec::VectorFunction> createMask(