diff --git a/src/de.hpi.swa.graal.squeak/src/de/hpi/swa/graal/squeak/model/ContextObject.java b/src/de.hpi.swa.graal.squeak/src/de/hpi/swa/graal/squeak/model/ContextObject.java
index c4472f0bb..8faebef5a 100644
--- a/src/de.hpi.swa.graal.squeak/src/de/hpi/swa/graal/squeak/model/ContextObject.java
+++ b/src/de.hpi.swa.graal.squeak/src/de/hpi/swa/graal/squeak/model/ContextObject.java
@@ -67,7 +67,12 @@ private ContextObject(final ContextObject original) {
         final int numStackSlots = code.getNumStackSlots();
         for (int i = 0; i < numStackSlots; i++) {
             final FrameSlot slot = code.getStackSlot(i);
-            FrameAccess.setStackSlot(truffleFrame, slot, original.truffleFrame.getValue(slot));
+            final Object value = original.truffleFrame.getValue(slot);
+            if (value != null) {
+                FrameAccess.setStackSlot(truffleFrame, slot, value);
+            } else {
+                break; // This and all following slots are not in use.
+            }
         }
     }
 
diff --git a/src/de.hpi.swa.graal.squeak/src/de/hpi/swa/graal/squeak/nodes/ExecuteContextNode.java b/src/de.hpi.swa.graal.squeak/src/de/hpi/swa/graal/squeak/nodes/ExecuteContextNode.java
index fccf53123..804cdeb9a 100644
--- a/src/de.hpi.swa.graal.squeak/src/de/hpi/swa/graal/squeak/nodes/ExecuteContextNode.java
+++ b/src/de.hpi.swa.graal.squeak/src/de/hpi/swa/graal/squeak/nodes/ExecuteContextNode.java
@@ -107,9 +107,6 @@ protected final Object doVirtualized(final VirtualFrame frame, @SuppressWarnings
     @Fallback
     protected final Object doNonVirtualized(final VirtualFrame frame, final ContextObject context) {
         // maybe persist newContext, so there's no need to lookup the context to update its pc.
-        assert code == context.getBlockOrMethod();
-        assert context.getMethod() == FrameAccess.getMethod(frame);
-        assert frame.getFrameDescriptor() == code.getFrameDescriptor();
         try {
             triggerInterruptHandlerNode.executeGeneric(frame, code.hasPrimitive(), bytecodeNodes.length);
             final long initialPC = context.getInstructionPointerForBytecodeLoop();
diff --git a/src/de.hpi.swa.graal.squeak/src/de/hpi/swa/graal/squeak/nodes/context/frame/FrameSlotWriteNode.java b/src/de.hpi.swa.graal.squeak/src/de/hpi/swa/graal/squeak/nodes/context/frame/FrameSlotWriteNode.java
index 36f3f2b44..e605be2cc 100644
--- a/src/de.hpi.swa.graal.squeak/src/de/hpi/swa/graal/squeak/nodes/context/frame/FrameSlotWriteNode.java
+++ b/src/de.hpi.swa.graal.squeak/src/de/hpi/swa/graal/squeak/nodes/context/frame/FrameSlotWriteNode.java
@@ -43,12 +43,10 @@ protected final void writeDouble(final Frame frame, final double value) {
 
     @Specialization(replaces = {"writeBool", "writeLong", "writeDouble"})
     protected final void writeObject(final Frame frame, final Object value) {
-        assert !(value instanceof Byte || value instanceof Integer || value instanceof Float) : "Illegal write operation";
-
         /* Initialize type on first write. No-op if kind is already Object. */
         frame.getFrameDescriptor().setFrameSlotKind(frameSlot, FrameSlotKind.Object);
 
-        assert value != null;
+        assert verifyWrite(value) : "Illegal write operation: " + value;
         frame.setObject(frameSlot, value);
     }
 
@@ -66,4 +64,8 @@ protected final boolean isDoubleOrIllegal(final Frame frame) {
         final FrameSlotKind kind = frame.getFrameDescriptor().getFrameSlotKind(frameSlot);
         return kind == FrameSlotKind.Double || kind == FrameSlotKind.Illegal;
     }
+
+    private static boolean verifyWrite(final Object value) {
+        return value != null && !(value instanceof Byte || value instanceof Integer || value instanceof Float);
+    }
 }
diff --git a/src/de.hpi.swa.graal.squeak/src/de/hpi/swa/graal/squeak/util/FrameAccess.java b/src/de.hpi.swa.graal.squeak/src/de/hpi/swa/graal/squeak/util/FrameAccess.java
index d75dac697..e39fc7314 100644
--- a/src/de.hpi.swa.graal.squeak/src/de/hpi/swa/graal/squeak/util/FrameAccess.java
+++ b/src/de.hpi.swa.graal.squeak/src/de/hpi/swa/graal/squeak/util/FrameAccess.java
@@ -190,13 +190,14 @@ public static void setStackPointer(final Frame frame, final CompiledCodeObject c
     /** Write to a frame slot (slow operation), prefer {@link FrameStackWriteNode}. */
     public static void setStackSlot(final Frame frame, final FrameSlot frameSlot, final Object value) {
         final FrameDescriptor frameDescriptor = frame.getFrameDescriptor();
-        if (value instanceof Boolean) {
+        final Class<? extends Object> valueClass = value.getClass();
+        if (valueClass == Boolean.class) {
             frameDescriptor.setFrameSlotKind(frameSlot, FrameSlotKind.Boolean);
             frame.setBoolean(frameSlot, (boolean) value);
-        } else if (value instanceof Long) {
+        } else if (valueClass == Long.class) {
             frameDescriptor.setFrameSlotKind(frameSlot, FrameSlotKind.Long);
             frame.setLong(frameSlot, (long) value);
-        } else if (value instanceof Double) {
+        } else if (valueClass == Double.class) {
             frameDescriptor.setFrameSlotKind(frameSlot, FrameSlotKind.Double);
             frame.setDouble(frameSlot, (double) value);
         } else {