diff --git a/src/eval.c b/src/eval.c
index f23d721..199dc9c 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -2769,10 +2769,11 @@
 	int	    vim9script = in_vim9script();
 
 	// "." is only string concatenation when scriptversion is 1
+	// "+=" and "-=" are assignment
 	p = eval_next_non_blank(*arg, evalarg, &getnext);
 	op = *p;
 	concat = op == '.' && (*(p + 1) == '.' || current_sctx.sc_version < 2);
-	if (op != '+' && op != '-' && !concat)
+	if ((op != '+' && op != '-' && !concat) || p[1] == '=')
 	    break;
 
 	evaluate = evalarg == NULL ? 0 : (evalarg->eval_flags & EVAL_EVALUATE);