diff --git a/flawfinder b/flawfinder
index e0704e1..7f1a80a 100755
--- a/flawfinder
+++ b/flawfinder
@@ -2000,6 +2000,9 @@ def process_options():
         print("*** getopt error:", text)
         usage()
         sys.exit(16)
+    if output_format == 1 and list_rules == 1:
+        print('You cannot list rules in HTML format')
+        sys.exit(20)
 
 
 def process_files():