diff --git a/src/prof.c b/src/prof.c
index fcf9c6f6..13334cb4 100644
--- a/src/prof.c
+++ b/src/prof.c
@@ -1303,6 +1303,7 @@ prof_dump_write(bool propagate_err, const char *s) {
 		prof_dump_buf_end += n;
 		i += n;
 	}
+	assert(i == slen);
 
 	return false;
 }