diff --git a/src/pkg/runtime/malloc.goc b/src/pkg/runtime/malloc.goc
index fbdd6bb021..4bea5e220c 100644
--- a/src/pkg/runtime/malloc.goc
+++ b/src/pkg/runtime/malloc.goc
@@ -60,7 +60,7 @@ runtime·mallocgc(uintptr size, uint32 flag, int32 dogc, int32 zeroed)
 		npages = size >> PageShift;
 		if((size & PageMask) != 0)
 			npages++;
-		s = runtime·MHeap_Alloc(&runtime·mheap, npages, 0, 1);
+		s = runtime·MHeap_Alloc(&runtime·mheap, npages, 0, 1, zeroed);
 		if(s == nil)
 			runtime·throw("out of memory");
 		size = npages<<PageShift;
diff --git a/src/pkg/runtime/malloc.h b/src/pkg/runtime/malloc.h
index 66919c911e..081ebd1394 100644
--- a/src/pkg/runtime/malloc.h
+++ b/src/pkg/runtime/malloc.h
@@ -380,7 +380,7 @@ struct MHeap
 extern MHeap runtime·mheap;
 
 void	runtime·MHeap_Init(MHeap *h, void *(*allocator)(uintptr));
-MSpan*	runtime·MHeap_Alloc(MHeap *h, uintptr npage, int32 sizeclass, int32 acct);
+MSpan*	runtime·MHeap_Alloc(MHeap *h, uintptr npage, int32 sizeclass, int32 acct, int32 zeroed);
 void	runtime·MHeap_Free(MHeap *h, MSpan *s, int32 acct);
 MSpan*	runtime·MHeap_Lookup(MHeap *h, void *v);
 MSpan*	runtime·MHeap_LookupMaybe(MHeap *h, void *v);
diff --git a/src/pkg/runtime/mcentral.c b/src/pkg/runtime/mcentral.c
index 6fc95aec7b..558b35b0e1 100644
--- a/src/pkg/runtime/mcentral.c
+++ b/src/pkg/runtime/mcentral.c
@@ -207,7 +207,7 @@ MCentral_Grow(MCentral *c)
 
 	runtime·unlock(c);
 	runtime·MGetSizeClassInfo(c->sizeclass, &size, &npages, &n);
-	s = runtime·MHeap_Alloc(&runtime·mheap, npages, c->sizeclass, 0);
+	s = runtime·MHeap_Alloc(&runtime·mheap, npages, c->sizeclass, 0, 1);
 	if(s == nil) {
 		// TODO(rsc): Log out of memory
 		runtime·lock(c);
diff --git a/src/pkg/runtime/mheap.c b/src/pkg/runtime/mheap.c
index 077217dc5d..a8a435b20e 100644
--- a/src/pkg/runtime/mheap.c
+++ b/src/pkg/runtime/mheap.c
@@ -66,7 +66,7 @@ runtime·MHeap_Init(MHeap *h, void *(*alloc)(uintptr))
 // Allocate a new span of npage pages from the heap
 // and record its size class in the HeapMap and HeapMapCache.
 MSpan*
-runtime·MHeap_Alloc(MHeap *h, uintptr npage, int32 sizeclass, int32 acct)
+runtime·MHeap_Alloc(MHeap *h, uintptr npage, int32 sizeclass, int32 acct, int32 zeroed)
 {
 	MSpan *s;
 
@@ -81,6 +81,8 @@ runtime·MHeap_Alloc(MHeap *h, uintptr npage, int32 sizeclass, int32 acct)
 		}
 	}
 	runtime·unlock(h);
+	if(s != nil && *(uintptr*)(s->start<<PageShift) != 0 && zeroed)
+		runtime·memclr((byte*)(s->start<<PageShift), s->npages<<PageShift);
 	return s;
 }
 
@@ -138,9 +140,6 @@ HaveSpan:
 		MHeap_FreeLocked(h, t);
 	}
 
-	if(*(uintptr*)(s->start<<PageShift) != 0)
-		runtime·memclr((byte*)(s->start<<PageShift), s->npages<<PageShift);
-
 	// Record span info, because gc needs to be
 	// able to map interior pointer to containing span.
 	s->sizeclass = sizeclass;