diff --git a/lib/command_lex.l b/lib/command_lex.l
index deec1757c2..c020d193a1 100644
--- a/lib/command_lex.l
+++ b/lib/command_lex.l
@@ -23,6 +23,9 @@
  */
 
 %{
+/* ignore harmless bug in old versions of flex */
+#pragma GCC diagnostic ignored "-Wsign-compare"
+
 #include "command_parse.h"
 
 #define YY_USER_ACTION yylloc->last_column += yyleng;