I'm going to risk embarrassing myself here because I know this is something really basic. But I'm just not seeing it.

In the following code, when I click Save, I get a DML exception saying that I did not include required fields Account__c and Invoice_Date__c, even though I entered them on the form. I've even tried initializing them in the constructor. But when it gets to the Save method all the properties in the "inv" variable are null.

I've also tried this with the format shown in the VF reference guide: "private final Invoice__c inv", using invoice__c in the markup. I've also tried doing this with a public getter method that returns inv. Nothing works.

WHY?!

//*** Page Markup ***

<apex:page standardController="Invoice__c" extensions="brokenInvoiceController" >
  <apex:form >

    <apex:pageBlock >
      <apex:pageMessages />
      <apex:pageBlockButtons >
        <apex:commandButton action="{!save}" value="Save" immediate="true" />
        <apex:commandButton action="{!cancel}" value="Cancel"/>
      </apex:pageBlockButtons>
      <apex:pageBlockSection >
        <apex:inputField value="{!inv.Account__c}" label="Account"/>
        <apex:inputField value="{!inv.Invoice_Date__c}" label="Invoice Date"/>
        <apex:inputField value="{!inv.Memo__c}" label="Memo" />
      </apex:pageBlockSection>
    </apex:pageBlock>
  </apex:form>
</apex:page>

//*** Controller Extension ***

public with sharing class brokenInvoiceController {

public Invoice__c inv {get;set;}

public brokenInvoiceController(ApexPages.StandardController controller) {
  inv = (Invoice__c)controller.getRecord();
}

public PageReference Save() {
  upsert inv;
  return null;
}

public PageReference Cancel() {
  return null;
}
}