diff --git a/nselib/strbuf.lua b/nselib/strbuf.lua
index 47c0a5252..7b251b02f 100644
--- a/nselib/strbuf.lua
+++ b/nselib/strbuf.lua
@@ -46,6 +46,7 @@ function concatbuf(sbuf, s)
   else
     error("bad #2 operand to strbuf concat operation", 2);
   end
+  return sbuf;
 end
 
 function eqbuf(sbuf1, sbuf2)