diff --git a/gdbstub.c b/gdbstub.c
index d54abd17cc..c41eb1de07 100644
--- a/gdbstub.c
+++ b/gdbstub.c
@@ -2064,7 +2064,11 @@ static void gdb_read_byte(GDBState *s, int ch)
             }
             break;
         case RS_GETLINE_RLE:
-            if (ch < ' ') {
+            /*
+             * Run-length encoding is explained in "Debugging with GDB /
+             * Appendix E GDB Remote Serial Protocol / Overview".
+             */
+            if (ch < ' ' || ch == '#' || ch == '$' || ch > 126) {
                 /* invalid RLE count encoding */
                 trace_gdbstub_err_invalid_repeat((uint8_t)ch);
                 s->state = RS_GETLINE;