diff --git a/pkt-line.c b/pkt-line.c
index eeb8046aca..116ef52c2d 100644
--- a/pkt-line.c
+++ b/pkt-line.c
@@ -97,10 +97,20 @@ void packet_buf_flush(struct strbuf *buf)
 	strbuf_add(buf, "0000", 4);
 }
 
-#define hex(a) (hexchar[(a) & 15])
-static void format_packet(struct strbuf *out, const char *fmt, va_list args)
+static void set_packet_header(char *buf, const int size)
 {
 	static char hexchar[] = "0123456789abcdef";
+
+	#define hex(a) (hexchar[(a) & 15])
+	buf[0] = hex(size >> 12);
+	buf[1] = hex(size >> 8);
+	buf[2] = hex(size >> 4);
+	buf[3] = hex(size);
+	#undef hex
+}
+
+static void format_packet(struct strbuf *out, const char *fmt, va_list args)
+{
 	size_t orig_len, n;
 
 	orig_len = out->len;
@@ -111,10 +121,7 @@ static void format_packet(struct strbuf *out, const char *fmt, va_list args)
 	if (n > LARGE_PACKET_MAX)
 		die("protocol error: impossibly long line");
 
-	out->buf[orig_len + 0] = hex(n >> 12);
-	out->buf[orig_len + 1] = hex(n >> 8);
-	out->buf[orig_len + 2] = hex(n >> 4);
-	out->buf[orig_len + 3] = hex(n);
+	set_packet_header(&out->buf[orig_len], n);
 	packet_trace(out->buf + orig_len + 4, n - 4, 1);
 }