diff --git a/extensions/xforms/nsXFormsMDGEngine.cpp b/extensions/xforms/nsXFormsMDGEngine.cpp
index 12f0831843db..0191377e3d78 100644
--- a/extensions/xforms/nsXFormsMDGEngine.cpp
+++ b/extensions/xforms/nsXFormsMDGEngine.cpp
@@ -367,7 +367,10 @@ nsXFormsMDGEngine::Recalculate(nsXFormsMDGSet* aChangedNodes)
         rv = xpath_res->GetStringValue(nodeval);
         NS_ENSURE_SUCCESS(rv, rv);
 
-        rv = SetNodeValue(g->mContextNode, nodeval, PR_FALSE, nsnull); 
+        rv = SetNodeValueInternal(g->mContextNode,
+                                  nodeval,
+                                  PR_FALSE,
+                                  PR_TRUE);
         if (NS_SUCCEEDED(rv)) {
           NS_ENSURE_TRUE(aChangedNodes->AddNode(g->mContextNode),
                          NS_ERROR_FAILURE);
@@ -609,8 +612,21 @@ nsXFormsMDGEngine::GetNodeValue(nsIDOMNode *aContextNode,
 nsresult
 nsXFormsMDGEngine::SetNodeValue(nsIDOMNode       *aContextNode,
                                 const nsAString  &aNodeValue,
-                                PRBool            aMarkNode,
                                 PRBool           *aNodeChanged)
+{
+  return SetNodeValueInternal(aContextNode,
+                              aNodeValue,
+                              PR_TRUE,
+                              PR_FALSE,
+                              aNodeChanged);
+}
+
+nsresult
+nsXFormsMDGEngine::SetNodeValueInternal(nsIDOMNode       *aContextNode,
+                                        const nsAString  &aNodeValue,
+                                        PRBool            aMarkNode,
+                                        PRBool            aIsCalculate,
+                                        PRBool           *aNodeChanged)
 {
   if (aNodeChanged) {
     *aNodeChanged = PR_FALSE;
@@ -619,7 +635,9 @@ nsXFormsMDGEngine::SetNodeValue(nsIDOMNode       *aContextNode,
   const nsXFormsNodeState* ns = GetNodeState(aContextNode);
   NS_ENSURE_TRUE(ns, NS_ERROR_FAILURE);
 
-  if (ns->IsReadonly()) {
+  // If the node is read-only and not set by a @calculate MIP,
+  // ignore the call
+  if (ns->IsReadonly() && !aIsCalculate) {
     ///
     /// @todo Better feedback for readonly nodes? (XXX)
     return NS_OK;
diff --git a/extensions/xforms/nsXFormsMDGEngine.h b/extensions/xforms/nsXFormsMDGEngine.h
index 8591447dbf29..045a4a32fcee 100644
--- a/extensions/xforms/nsXFormsMDGEngine.h
+++ b/extensions/xforms/nsXFormsMDGEngine.h
@@ -313,6 +313,20 @@ protected:
    */
   nsresult Invalidate();
   
+  /**
+   * Set the value of a node.
+
+   * @param aContextNode     The node to set the value for
+   * @param aNodeValue       The value
+   * @param aMarkNode        Whether to mark node as changed
+   * @param aNodeChanged     Was node changed?
+   * @param aIsCalculate     Is it a @calculate setting the value?
+   */
+  nsresult SetNodeValueInternal(nsIDOMNode      *aContextNode,
+                                const nsAString &aNodeValue,
+                                PRBool           aMarkNode = PR_TRUE,
+                                PRBool           aIsCalculate = PR_FALSE,
+                                PRBool          *aNodeChanged = nsnull);
 public:
   /**
    * Constructor
@@ -382,16 +396,14 @@ public:
   nsresult MarkNodeAsChanged(nsIDOMNode *aContextNode);
 
   /**
-   * Set the value of a node. (used by nsXFormsMDG)
-
+   * Set the value of a node -- the public version of SetNodeValueInternal().
+   *
    * @param aContextNode     The node to set the value for
    * @param aNodeValue       The value
-   * @param aMarkNode        Whether to mark node as changed
    * @param aNodeChanged     Was node changed?
    */
   nsresult SetNodeValue(nsIDOMNode      *aContextNode,
                         const nsAString &aNodeValue,
-                        PRBool           aMarkNode = PR_TRUE,
                         PRBool          *aNodeChanged = nsnull);
 
   /**
diff --git a/extensions/xforms/nsXFormsModelElement.cpp b/extensions/xforms/nsXFormsModelElement.cpp
index 33d62f63bdfd..e85a9c5e8a79 100644
--- a/extensions/xforms/nsXFormsModelElement.cpp
+++ b/extensions/xforms/nsXFormsModelElement.cpp
@@ -841,7 +841,6 @@ nsXFormsModelElement::SetNodeValue(nsIDOMNode      *aContextNode,
 { 
   return mMDG.SetNodeValue(aContextNode,
                            aNodeValue,
-                           PR_TRUE,
                            aNodeChanged);
 }