diff --git a/src/prof.c b/src/prof.c
index a4e30f42..4ebe2799 100644
--- a/src/prof.c
+++ b/src/prof.c
@@ -1292,7 +1292,7 @@ prof_dump_write(bool propagate_err, const char *s) {
 			}
 		}
 
-		if (prof_dump_buf_end + slen <= PROF_DUMP_BUFSIZE) {
+		if (prof_dump_buf_end + slen - i <= PROF_DUMP_BUFSIZE) {
 			/* Finish writing. */
 			n = slen - i;
 		} else {