diff --git a/mps/code/prmcxc.c b/mps/code/prmcxc.c
index d521dcb96ca..315f1d31c9f 100644
--- a/mps/code/prmcxc.c
+++ b/mps/code/prmcxc.c
@@ -24,19 +24,39 @@ SRCID(prmcxc, "$Id$");
 Bool MutatorContextCheck(MutatorContext context)
 {
   CHECKS(MutatorContext, context);
+  CHECKL(sizeof *context->threadState == sizeof(THREAD_STATE_S));
+  CHECKL(NONNEGATIVE(context->var));
+  CHECKL(context->var < MutatorContextLIMIT);
+  CHECKL((context->var == MutatorContextTHREAD) == (context->address == NULL));
   CHECKL(context->threadState != NULL);
   return TRUE;
 }
 
 
-void MutatorContextInit(MutatorContext context, Addr address,
-                        THREAD_STATE_S *threadState)
+void MutatorContextInitFault(MutatorContext context, Addr address,
+                             THREAD_STATE_S *threadState)
+{
+  AVER(context != NULL);
+  AVER(address != NULL);
+  AVER(threadState != NULL);
+
+  context->var = MutatorContextFAULT;
+  context->address = address;
+  context->threadState = threadState;
+  context->sig = MutatorContextSig;
+
+  AVERT(MutatorContext, context);
+}
+
+
+void MutatorContextInitThread(MutatorContext context,
+                              THREAD_STATE_S *threadState)
 {
   AVER(context != NULL);
   AVER(threadState != NULL);
 
-  context->address = address;
-  AVER(sizeof *context->threadState == sizeof(THREAD_STATE_S));
+  context->var = MutatorContextTHREAD;
+  context->address = NULL;
   context->threadState = threadState;
   context->sig = MutatorContextSig;
 
@@ -50,7 +70,9 @@ Res MutatorContextScan(ScanState ss, MutatorContext context,
   THREAD_STATE_S *mc;
   Res res;
 
+  AVERT(ScanState, ss);
   AVERT(MutatorContext, context);
+  AVER(context->var == MutatorContextTHREAD);
 
   /* This scans the root registers (.context.regroots).  It also
      unnecessarily scans the rest of the context.  The optimisation
diff --git a/mps/code/prmcxc.h b/mps/code/prmcxc.h
index 832b0c4015a..896fa87313c 100644
--- a/mps/code/prmcxc.h
+++ b/mps/code/prmcxc.h
@@ -16,13 +16,16 @@
 
 typedef struct MutatorContextStruct {
   Sig sig;                      /* <design/sig/> */
-  Addr address;
+  MutatorContextVar var;        /* Discriminator. */
+  Addr address;                 /* Fault address, if stopped by protection
+                                 * fault; NULL if stopped by thread manager. */
   THREAD_STATE_S *threadState;
   /* FIXME: Might need to get the floats in case the compiler stashes
      intermediate values in them. */
 } MutatorContextStruct;
 
-extern void MutatorContextInit(MutatorContext context, Addr address, THREAD_STATE_S *threadState);
+extern void MutatorContextInitFault(MutatorContext context, Addr address, THREAD_STATE_S *threadState);
+extern void MutatorContextInitThread(MutatorContext context, THREAD_STATE_S *threadState);
 
 #endif /* prmcxc_h */
 
diff --git a/mps/code/prmcxci3.c b/mps/code/prmcxci3.c
index 1e51dec7aae..853b28a70f1 100644
--- a/mps/code/prmcxci3.c
+++ b/mps/code/prmcxci3.c
@@ -74,6 +74,7 @@ void Prmci3DecodeFaultContext(MRef *faultmemReturn,
   AVER(faultmemReturn != NULL);
   AVER(insvecReturn != NULL);
   AVERT(MutatorContext, context);
+  AVER(context->var == MutatorContextFAULT);
 
   *faultmemReturn = (MRef)context->address;
   *insvecReturn = (Byte*)context->threadState->__eip;
diff --git a/mps/code/prmcxci6.c b/mps/code/prmcxci6.c
index 1c7d6ac5f91..f16967b0f85 100644
--- a/mps/code/prmcxci6.c
+++ b/mps/code/prmcxci6.c
@@ -79,6 +79,7 @@ void Prmci6DecodeFaultContext(MRef *faultmemReturn,
   AVER(faultmemReturn != NULL);
   AVER(insvecReturn != NULL);
   AVERT(MutatorContext, context);
+  AVER(context->var == MutatorContextFAULT);
 
   *faultmemReturn = (MRef)context->address;
   *insvecReturn = (Byte*)context->threadState->__rip;
diff --git a/mps/code/protxc.c b/mps/code/protxc.c
index 2674fd0610e..3edbfd8ddb4 100644
--- a/mps/code/protxc.c
+++ b/mps/code/protxc.c
@@ -238,8 +238,8 @@ static void protCatchOne(void)
 
     /* The cast via Word suppresses "cast to pointer from integer of
        different size" warnings in GCC, for the  XCI3GC build. */
-    MutatorContextInit(&context, (Addr)(Word)request.code[1],
-                       (void *)request.old_state);
+    MutatorContextInitFault(&context, (Addr)(Word)request.code[1],
+                            (void *)request.old_state);
   
     if (ArenaAccess(context.address,
                     AccessREAD | AccessWRITE,
diff --git a/mps/code/thxc.c b/mps/code/thxc.c
index 0952c75fa08..3ef92e63aa6 100644
--- a/mps/code/thxc.c
+++ b/mps/code/thxc.c
@@ -236,7 +236,7 @@ Res ThreadScan(ScanState ss, Thread thread, Word *stackCold,
        order to assert that the thread is suspended, but it's probably
        unnecessary and is a lot of work to check a static condition. */
 
-    MutatorContextInit(&context, NULL, &threadState);
+    MutatorContextInitThread(&context, &threadState);
 
     count = THREAD_STATE_COUNT;
     AVER(sizeof(*context.threadState) == count * sizeof(natural_t));