diff --git a/src/main/java/walkingkooka/tree/expression/function/ExpressionFunctionParameter.java b/src/main/java/walkingkooka/tree/expression/function/ExpressionFunctionParameter.java
index e2ea79e6..dadb7022 100644
--- a/src/main/java/walkingkooka/tree/expression/function/ExpressionFunctionParameter.java
+++ b/src/main/java/walkingkooka/tree/expression/function/ExpressionFunctionParameter.java
@@ -90,7 +90,7 @@ public static <T> ExpressionFunctionParameter<T> with(final ExpressionFunctionPa
         checkName(name);
         checkType(type);
         checkTypeParameters(typeParameters);
-        Objects.requireNonNull(cardinality, "cardinality");
+        checkCardinality(cardinality);
         Objects.requireNonNull(kinds, "kinds");
 
         return new ExpressionFunctionParameter<>(
@@ -114,6 +114,10 @@ private static List<Class<?>> checkTypeParameters(final List<Class<?>> typeParam
         return Objects.requireNonNull(typeParameters, "typeParameters");
     }
 
+    private static ExpressionFunctionParameterCardinality checkCardinality(final ExpressionFunctionParameterCardinality cardinality) {
+        return Objects.requireNonNull(cardinality, "cardinality");
+    }
+
     private ExpressionFunctionParameter(final ExpressionFunctionParameterName name,
                                         final Class<T> type,
                                         final ExpressionFunctionParameterCardinality cardinality,
@@ -210,7 +214,7 @@ public ExpressionFunctionParameterCardinality cardinality() {
      * Would be setter that returns a {@link ExpressionFunctionParameter} with the given {@link ExpressionFunctionParameterCardinality}.
      */
     public ExpressionFunctionParameter<T> setCardinality(final ExpressionFunctionParameterCardinality cardinality) {
-        Objects.requireNonNull(cardinality, "cardinality");
+        checkCardinality(cardinality);
 
         return this.cardinality.equals(cardinality) ?
                 this :