diff --git a/addrcheck/ac_main.c b/addrcheck/ac_main.c
index e93981aad..bcb937a59 100644
--- a/addrcheck/ac_main.c
+++ b/addrcheck/ac_main.c
@@ -986,7 +986,8 @@ void ac_fpu_ACCESS_check_SLOWLY ( Addr addr, SizeT size, Bool isWrite )
 /*--- Our instrumenter                                     ---*/
 /*------------------------------------------------------------*/
 
-IRBB* TL_(instrument)(IRBB* bb_in, VexGuestLayout* layout, IRType hWordTy )
+IRBB* TL_(instrument)(IRBB* bb_in, VexGuestLayout* layout, 
+                      IRType gWordTy, IRType hWordTy )
 {
    Int         i, hsz;
    IRStmt*     st;
@@ -995,9 +996,15 @@ IRBB* TL_(instrument)(IRBB* bb_in, VexGuestLayout* layout, IRType hWordTy )
    IRExpr*     guard;
    IRDirty*    di;
    Bool        isLoad;
+   IRBB*       bb;
+
+   if (gWordTy != hWordTy) {
+      /* We don't currently support this case. */
+      VG_(tool_panic)("host/guest word size mismatch");
+   }
 
    /* Set up BB */
-   IRBB* bb     = emptyIRBB();
+   bb           = emptyIRBB();
    bb->tyenv    = dopyIRTypeEnv(bb_in->tyenv);
    bb->next     = dopyIRExpr(bb_in->next);
    bb->jumpkind = bb_in->jumpkind;
diff --git a/cachegrind/cg_main.c b/cachegrind/cg_main.c
index d6e18ee2e..40fd86c3c 100644
--- a/cachegrind/cg_main.c
+++ b/cachegrind/cg_main.c
@@ -559,7 +559,8 @@ void endOfInstr(IRBB* bbOut, instr_info* i_node, Bool bbSeenBefore,
    addStmtToIRBB( bbOut, IRStmt_Dirty(di) );
 }
 
-IRBB* TL_(instrument) ( IRBB* bbIn, VexGuestLayout* layout, IRType hWordTy )
+IRBB* TL_(instrument) ( IRBB* bbIn, VexGuestLayout* layout, 
+                        IRType gWordTy, IRType hWordTy )
 {
    Int      i, dataSize = 0, bbInfo_i;
    IRBB*    bbOut;
@@ -570,6 +571,11 @@ IRBB* TL_(instrument) ( IRBB* bbIn, VexGuestLayout* layout, IRType hWordTy )
    UInt     instrLen;
    IRExpr  *loadAddrExpr, *storeAddrExpr;
 
+   if (gWordTy != hWordTy) {
+      /* We don't currently support this case. */
+      VG_(tool_panic)("host/guest word size mismatch");
+   }
+
    /* Set up BB */
    bbOut           = emptyIRBB();
    bbOut->tyenv    = dopyIRTypeEnv(bbIn->tyenv);
diff --git a/corecheck/cc_main.c b/corecheck/cc_main.c
index 260157087..0d815120b 100644
--- a/corecheck/cc_main.c
+++ b/corecheck/cc_main.c
@@ -51,7 +51,8 @@ void TL_(post_clo_init)(void)
 {
 }
 
-IRBB* TL_(instrument)(IRBB* bb_in, VexGuestLayout* layout, IRType hWordTy )
+IRBB* TL_(instrument)(IRBB* bb_in, VexGuestLayout* layout, 
+                      IRType gWordTy, IRType hWordTy )
 {
     return bb_in;
 }
diff --git a/coregrind/toolfuncs.def b/coregrind/toolfuncs.def
index 1806c3bcb..4fa66dec5 100644
--- a/coregrind/toolfuncs.def
+++ b/coregrind/toolfuncs.def
@@ -26,7 +26,7 @@ void,		post_clo_init
 ## Instrument a basic block.  Must be a true function, ie. the same input
 ## always results in the same output, because basic blocks can be
 ## retranslated.  Unless you're doing something really strange...
-IRBB*,  instrument,  IRBB* bb,  VexGuestLayout* layout,  IRType hWordTy
+IRBB*, instrument, IRBB* bb, VexGuestLayout* layout, IRType gWordTy, IRType hWordTy
 
 ## Finish up, print out any results, etc.  `exitcode' is program's exit
 ## code.  The shadow can be found with VG_(get_exit_status_shadow)().
diff --git a/lackey/lk_main.c b/lackey/lk_main.c
index ef05d90c6..55fdc7e1c 100644
--- a/lackey/lk_main.c
+++ b/lackey/lk_main.c
@@ -129,13 +129,20 @@ void TL_(post_clo_init)(void)
    Which gives us the right answer.  And just to avoid two C calls, we fold
    the basic-block-beginning call in with add_one_BB().  Phew.
 */ 
-IRBB* TL_(instrument)(IRBB* bb_in, VexGuestLayout* layout, IRType hWordTy )
+IRBB* TL_(instrument)(IRBB* bb_in, VexGuestLayout* layout, 
+                      IRType gWordTy, IRType hWordTy )
 {
    IRDirty* di;
    Int      i;
+   IRBB*    bb;
+
+   if (gWordTy != hWordTy) {
+      /* We don't currently support this case. */
+      VG_(tool_panic)("host/guest word size mismatch");
+   }
 
    /* Set up BB */
-   IRBB* bb     = emptyIRBB();
+   bb           = emptyIRBB();
    bb->tyenv    = dopyIRTypeEnv(bb_in->tyenv);
    bb->next     = dopyIRExpr(bb_in->next);
    bb->jumpkind = bb_in->jumpkind;
diff --git a/massif/ms_main.c b/massif/ms_main.c
index 9ce0407df..1576b5fe6 100644
--- a/massif/ms_main.c
+++ b/massif/ms_main.c
@@ -1210,8 +1210,10 @@ void TL_(post_clo_init)(void)
 /*--- Instrumentation                                      ---*/
 /*------------------------------------------------------------*/
 
-IRBB* TL_(instrument) ( IRBB* bb_in, VexGuestLayout* layout, IRType hWordTy )
+IRBB* TL_(instrument) ( IRBB* bb_in, VexGuestLayout* layout, 
+                        IRType gWordTy, IRType hWordTy )
 {
+   /* XXX Will Massif work when gWordTy != hWordTy ? */
    return bb_in;
 }
 
diff --git a/memcheck/mc_translate.c b/memcheck/mc_translate.c
index 5dc79be61..f6f984b03 100644
--- a/memcheck/mc_translate.c
+++ b/memcheck/mc_translate.c
@@ -2383,16 +2383,23 @@ static Bool checkForBogusLiterals ( /*FLAT*/ IRStmt* st )
 }
 
 
-IRBB* TL_(instrument) ( IRBB* bb_in, VexGuestLayout* layout, IRType hWordTy )
+IRBB* TL_(instrument) ( IRBB* bb_in, VexGuestLayout* layout, 
+                        IRType gWordTy, IRType hWordTy )
 {
    Bool verboze = False; //True; 
 
    Int     i, j, first_stmt;
    IRStmt* st;
    MCEnv   mce;
+   IRBB*   bb;
+
+   if (gWordTy != hWordTy) {
+      /* We don't currently support this case. */
+      VG_(tool_panic)("host/guest word size mismatch");
+   }
 
    /* Set up BB */
-   IRBB* bb     = emptyIRBB();
+   bb           = emptyIRBB();
    bb->tyenv    = dopyIRTypeEnv(bb_in->tyenv);
    bb->next     = dopyIRExpr(bb_in->next);
    bb->jumpkind = bb_in->jumpkind;
diff --git a/none/nl_main.c b/none/nl_main.c
index c2fcb584f..2f136c2c6 100644
--- a/none/nl_main.c
+++ b/none/nl_main.c
@@ -46,7 +46,8 @@ void TL_(post_clo_init)(void)
 {
 }
 
-IRBB* TL_(instrument)(IRBB* bb, VexGuestLayout* layout, IRType hWordTy)
+IRBB* TL_(instrument)(IRBB* bb, VexGuestLayout* layout, 
+                      IRType gWordTy, IRType hWordTy)
 {
     return bb;
 }