diff --git a/javalib/src/android/system/virtualmachine/VirtualMachine.java b/javalib/src/android/system/virtualmachine/VirtualMachine.java
index 0e549ae..7176486 100644
--- a/javalib/src/android/system/virtualmachine/VirtualMachine.java
+++ b/javalib/src/android/system/virtualmachine/VirtualMachine.java
@@ -112,6 +112,9 @@
     /* package */ static VirtualMachine create(
             Context context, String name, VirtualMachineConfig config)
             throws VirtualMachineException {
+        if (config == null) {
+            throw new VirtualMachineException("null config");
+        }
         VirtualMachine vm = new VirtualMachine(context, name, config);
 
         try {
@@ -331,6 +334,10 @@
         if (!oldConfig.isCompatibleWith(newConfig)) {
             throw new VirtualMachineException("incompatible config");
         }
+        if (getStatus() != Status.STOPPED) {
+            throw new VirtualMachineException(
+                    "can't change config while virtual machine is not stopped");
+        }
 
         try {
             FileOutputStream output = new FileOutputStream(mConfigFilePath);