diff --git a/src/vim9expr.c b/src/vim9expr.c
index 69f670b..efa2ef5 100644
--- a/src/vim9expr.c
+++ b/src/vim9expr.c
@@ -2513,7 +2513,8 @@
 		if (may_generate_2STRING(-2, FALSE, cctx) == FAIL
 			|| may_generate_2STRING(-1, FALSE, cctx) == FAIL)
 		    return FAIL;
-		generate_instr_drop(cctx, ISN_CONCAT, 1);
+		if (generate_CONCAT(cctx, 2) == FAIL)
+		    return FAIL;
 	    }
 	    else
 		generate_two_op(cctx, op);