diff --git a/CHERICC_Fat.bsv b/CHERICC_Fat.bsv
index 1470684..7b25ecd 100644
--- a/CHERICC_Fat.bsv
+++ b/CHERICC_Fat.bsv
@@ -1078,7 +1078,7 @@ instance CHERICap #(CapReg, OTypeW, FlagsW, CapAddressW, CapW, TSub#(MW, 3));
 
   function Bool isDerivable (CapReg cap);
     return cap.bounds.exp <= resetExp
-        && !(cap.bounds.exp == resetExp && (cap.bounds.topBits != 0) || (cap.bounds.baseBits[valueOf(TSub#(MW,1))] != 1'b0))
+        && !(cap.bounds.exp == resetExp && ((truncateLSB(cap.bounds.topBits) != 1'b0) || (cap.bounds.baseBits[valueOf(TSub#(MW,1))] != 1'b0)))
         && !(cap.bounds.exp == resetExp-1 && (cap.bounds.baseBits[valueOf(TSub#(MW,1)):valueOf(TSub#(MW,2))] != 2'b0))
         && (cap.reserved == 0);
   endfunction