diff --git a/macros.js b/macros.js
index 8639682..110ec62 100644
--- a/macros.js
+++ b/macros.js
@@ -328,9 +328,9 @@ function busToString(busname){
 	if(busname=='State')
 		return listActiveTCStates();
 	if(busname=='Execute')
-		return dis6502[readBits('ir',8)].replace(/ /,' ');
+		return dis6502toHTML(readBits('ir',8));
 	if(busname=='Fetch')
-		return isNodeHigh(nodenames['sync'])?dis6502[readDataBus()].replace(/ /,' '):'';
+		return isNodeHigh(nodenames['sync'])?dis6502toHTML(readDataBus()):"";
 	if(busname=='plaOutputs')
 		// PLA outputs are mostly ^op- but some have a prefix too
 		//    - we'll allow the x and xx prefix but ignore the #
@@ -622,6 +622,14 @@ function adler32(x){
 	return (0x100000000+(b<<16)+a).toString(16).slice(-8);
 }
 
+// sanitised opcode for HTML output
+function dis6502toHTML(byte){
+	var opcode=dis6502[byte];
+	if(typeof opcode == "undefined")
+		return "unknown"
+	return opcode.replace(/ /,'&nbsp;');
+}
+
 // opcode lookup for 6502 - not quite a disassembly
 //   javascript derived from Debugger.java by Achim Breidenbach
 var dis6502={