diff --git a/src/pkg/runtime/os_netbsd.c b/src/pkg/runtime/os_netbsd.c
index 9c0511d4c00..95c1070d1b2 100644
--- a/src/pkg/runtime/os_netbsd.c
+++ b/src/pkg/runtime/os_netbsd.c
@@ -94,7 +94,7 @@ runtime·semasleep(int64 ns)
 				runtime·atomicstore(&m->waitsemalock, 0);
 				runtime·lwp_park(nil, 0, &m->waitsemacount, nil);
 			} else {
-				ns += runtime·nanotime();
+				ns = ns + runtime·nanotime();
 				// NOTE: tv_nsec is int64 on amd64, so this assumes a little-endian system.
 				ts.tv_nsec = 0;
 				ts.tv_sec = runtime·timediv(ns, 1000000000, (int32*)&ts.tv_nsec);