diff --git a/internal/config/types.go b/internal/config/types.go
index fd830e983..c2296975f 100644
--- a/internal/config/types.go
+++ b/internal/config/types.go
@@ -43,8 +43,8 @@ func (p *IPPrefixes) String() string {
 	var buf byteutil.Buffer
 	for _, prefix := range *p {
 		str := prefix.String()
-		buf.WriteString(str)
-		buf.WriteByte(',')
+		buf.B = append(buf.B, str...)
+		buf.B = append(buf.B, ',')
 	}
 	buf.Truncate(1)
 	return buf.String()