diff --git a/src/share/classes/java/io/ObjectInputStream.java b/src/share/classes/java/io/ObjectInputStream.java
index ca0400539f96323231fbc0083728f1919d44fe3b..0e41b08dc3ee2a23362f9cd70b718ec19bc3e58c 100644
--- a/src/share/classes/java/io/ObjectInputStream.java
+++ b/src/share/classes/java/io/ObjectInputStream.java
@@ -490,11 +490,12 @@ public class ObjectInputStream
     public void defaultReadObject()
         throws IOException, ClassNotFoundException
     {
-        if (curContext == null) {
+        SerialCallbackContext ctx = curContext;
+        if (ctx == null) {
             throw new NotActiveException("not in call to readObject");
         }
-        Object curObj = curContext.getObj();
-        ObjectStreamClass curDesc = curContext.getDesc();
+        Object curObj = ctx.getObj();
+        ObjectStreamClass curDesc = ctx.getDesc();
         bin.setBlockDataMode(false);
         defaultReadFields(curObj, curDesc);
         bin.setBlockDataMode(true);
@@ -528,11 +529,12 @@ public class ObjectInputStream
     public ObjectInputStream.GetField readFields()
         throws IOException, ClassNotFoundException
     {
-        if (curContext == null) {
+        SerialCallbackContext ctx = curContext;
+        if (ctx == null) {
             throw new NotActiveException("not in call to readObject");
         }
-        Object curObj = curContext.getObj();
-        ObjectStreamClass curDesc = curContext.getDesc();
+        Object curObj = ctx.getObj();
+        ObjectStreamClass curDesc = ctx.getDesc();
         bin.setBlockDataMode(false);
         GetFieldImpl getField = new GetFieldImpl(curDesc);
         getField.readFields();
@@ -1967,7 +1969,6 @@ public class ObjectInputStream
     private void defaultReadFields(Object obj, ObjectStreamClass desc)
         throws IOException
     {
-        // REMIND: is isInstance check necessary?
         Class<?> cl = desc.forClass();
         if (cl != null && obj != null && !cl.isInstance(obj)) {
             throw new ClassCastException();
diff --git a/src/share/classes/java/io/ObjectOutputStream.java b/src/share/classes/java/io/ObjectOutputStream.java
index f7a94bb0342a0cf394341a435077353120a60faa..c99aeb9cd67b53169b38b1d005d0bb2d0ca25c97 100644
--- a/src/share/classes/java/io/ObjectOutputStream.java
+++ b/src/share/classes/java/io/ObjectOutputStream.java
@@ -430,11 +430,12 @@ public class ObjectOutputStream
      *          <code>OutputStream</code>
      */
     public void defaultWriteObject() throws IOException {
-        if ( curContext == null ) {
+        SerialCallbackContext ctx = curContext;
+        if (ctx == null) {
             throw new NotActiveException("not in call to writeObject");
         }
-        Object curObj = curContext.getObj();
-        ObjectStreamClass curDesc = curContext.getDesc();
+        Object curObj = ctx.getObj();
+        ObjectStreamClass curDesc = ctx.getDesc();
         bout.setBlockDataMode(false);
         defaultWriteFields(curObj, curDesc);
         bout.setBlockDataMode(true);
@@ -452,11 +453,12 @@ public class ObjectOutputStream
      */
     public ObjectOutputStream.PutField putFields() throws IOException {
         if (curPut == null) {
-            if (curContext == null) {
+            SerialCallbackContext ctx = curContext;
+            if (ctx == null) {
                 throw new NotActiveException("not in call to writeObject");
             }
-            Object curObj = curContext.getObj();
-            ObjectStreamClass curDesc = curContext.getDesc();
+            Object curObj = ctx.getObj();
+            ObjectStreamClass curDesc = ctx.getDesc();
             curPut = new PutFieldImpl(curDesc);
         }
         return curPut;
@@ -1516,7 +1518,11 @@ public class ObjectOutputStream
     private void defaultWriteFields(Object obj, ObjectStreamClass desc)
         throws IOException
     {
-        // REMIND: perform conservative isInstance check here?
+        Class<?> cl = desc.forClass();
+        if (cl != null && obj != null && !cl.isInstance(obj)) {
+            throw new ClassCastException();
+        }
+
         desc.checkDefaultSerialize();
 
         int primDataSize = desc.getPrimDataSize();