diff --git a/src/main/java/walkingkooka/tree/expression/function/ExpressionFunctionParameter.java b/src/main/java/walkingkooka/tree/expression/function/ExpressionFunctionParameter.java
index dadb7022..d68b52d7 100644
--- a/src/main/java/walkingkooka/tree/expression/function/ExpressionFunctionParameter.java
+++ b/src/main/java/walkingkooka/tree/expression/function/ExpressionFunctionParameter.java
@@ -91,7 +91,7 @@ public static <T> ExpressionFunctionParameter<T> with(final ExpressionFunctionPa
         checkType(type);
         checkTypeParameters(typeParameters);
         checkCardinality(cardinality);
-        Objects.requireNonNull(kinds, "kinds");
+        checkKinds(kinds);
 
         return new ExpressionFunctionParameter<>(
                 name,
@@ -118,6 +118,10 @@ private static ExpressionFunctionParameterCardinality checkCardinality(final Exp
         return Objects.requireNonNull(cardinality, "cardinality");
     }
 
+    private static Set<ExpressionFunctionParameterKind> checkKinds(final Set<ExpressionFunctionParameterKind> kinds) {
+        return Objects.requireNonNull(kinds, "kinds");
+    }
+
     private ExpressionFunctionParameter(final ExpressionFunctionParameterName name,
                                         final Class<T> type,
                                         final ExpressionFunctionParameterCardinality cardinality,
@@ -240,7 +244,7 @@ public Set<ExpressionFunctionParameterKind> kinds() {
      * Returns a {@link ExpressionFunctionParameter} with the given {@link ExpressionFunctionParameterKind}.
      */
     public ExpressionFunctionParameter<T> setKinds(final Set<ExpressionFunctionParameterKind> kinds) {
-        Objects.requireNonNull(kinds, "kinds");
+        checkKinds(kinds);
 
         final Set<ExpressionFunctionParameterKind> copy = kinds.isEmpty() ?
                 Sets.empty() :