diff --git a/picorv32.v b/picorv32.v
index cfc7ce0..0104eff 100644
--- a/picorv32.v
+++ b/picorv32.v
@@ -698,7 +698,7 @@ module picorv32 #(
 	`FORMAL_KEEP reg dbg_rs2val_valid;
 
 	always @* begin
-		new_ascii_instr = "";
+		new_ascii_instr = "UNDEFINED";
 
 		if (instr_lui)      new_ascii_instr = "lui";
 		if (instr_auipc)    new_ascii_instr = "auipc";