src/pkg/runtime/mgc0.c | 5 ++++-

diff --git a/src/pkg/runtime/mgc0.c b/src/pkg/runtime/mgc0.c
index 4b2108ba7a12c04fc7ffd779986630a6effa6aea..761f128a85afb44ed5f52e4fc4153bde974479c0 100644
--- a/src/pkg/runtime/mgc0.c
+++ b/src/pkg/runtime/mgc0.c
@@ -1798,6 +1798,8 @@
 void
 runtime·gchelper(void)
 {
+	int32 nproc;
+
 	gchelperstart();
 
 	// parallel mark for over gc roots
@@ -1814,7 +1816,8 @@ 	}
 
 	runtime·parfordo(work.sweepfor);
 	bufferList[m->helpgc].busy = 0;
-	if(runtime·xadd(&work.ndone, +1) == work.nproc-1)
+	nproc = work.nproc;  // work.nproc can change right after we increment work.ndone
+	if(runtime·xadd(&work.ndone, +1) == nproc-1)
 		runtime·notewakeup(&work.alldone);
 }