diff --git a/lib/seq_buf.c b/lib/seq_buf.c
index 11f2ae0..bd807f5 100644
--- a/lib/seq_buf.c
+++ b/lib/seq_buf.c
@@ -140,13 +140,17 @@
  */
 int seq_buf_puts(struct seq_buf *s, const char *str)
 {
-	unsigned int len = strlen(str);
+	size_t len = strlen(str);
 
 	WARN_ON(s->size == 0);
 
+	/* Add 1 to len for the trailing null byte which must be there */
+	len += 1;
+
 	if (seq_buf_can_fit(s, len)) {
 		memcpy(s->buffer + s->len, str, len);
-		s->len += len;
+		/* Don't count the trailing null byte against the capacity */
+		s->len += len - 1;
 		return 0;
 	}
 	seq_buf_set_overflow(s);