diff --git a/CHERICC_Fat.bsv b/CHERICC_Fat.bsv
index 26658e6..d732329 100644
--- a/CHERICC_Fat.bsv
+++ b/CHERICC_Fat.bsv
@@ -1021,8 +1021,9 @@ instance CHERICap #(CapReg, OTypeW, FlagsW, CapAddressW, CapW, TSub#(MW, 3));
 
   function CapReg maskAddr (CapReg cap, Bit#(TSub#(MW, 3)) mask);
     cap.address[valueOf(TSub#(MW, 4)):0] = cap.address[valueOf(TSub#(MW, 4)):0] & mask;
+    UInt#(TAdd#(ExpW,1)) expExt = zeroExtend(cap.bounds.exp);
     //Update addrBits. Since exp can be up to 64, extend to 64 + 8 bits so bit-select is always in range
-    cap.addrBits = (({40'b0,cap.address})[cap.bounds.exp+fromInteger(valueOf(TSub#(MW,1))):cap.bounds.exp]); //TODO avoid shift?
+    cap.addrBits = (({40'b0,cap.address})[expExt+fromInteger(valueOf(TSub#(MW,1))):expExt]); //TODO avoid shift?
     return cap;
   endfunction