diff --git a/rtl/debug/jtag_dm.v b/rtl/debug/jtag_dm.v
index 985e62d..6463dd0 100644
--- a/rtl/debug/jtag_dm.v
+++ b/rtl/debug/jtag_dm.v
@@ -144,6 +144,9 @@ module jtag_dm #(
                     state_next = STATE_RESP;
                 end
             end
+            default: begin
+                state_next = STATE_IDLE;
+            end
         endcase
     end