diff --git a/src/runtime/mheap.go b/src/runtime/mheap.go
index b7c5add40c..558ff1f689 100644
--- a/src/runtime/mheap.go
+++ b/src/runtime/mheap.go
@@ -1327,8 +1327,11 @@ func (h *mheap) grow(npage uintptr) bool {
 	ask := alignUp(npage, pallocChunkPages) * pageSize
 
 	totalGrowth := uintptr(0)
-	nBase := alignUp(h.curArena.base+ask, physPageSize)
-	if nBase > h.curArena.end {
+	// This may overflow because ask could be very large
+	// and is otherwise unrelated to h.curArena.base.
+	end := h.curArena.base + ask
+	nBase := alignUp(end, physPageSize)
+	if nBase > h.curArena.end || /* overflow */ end < h.curArena.base {
 		// Not enough room in the current arena. Allocate more
 		// arena space. This may not be contiguous with the
 		// current arena, so we have to request the full ask.
@@ -1364,7 +1367,10 @@ func (h *mheap) grow(npage uintptr) bool {
 		mSysStatInc(&memstats.heap_released, asize)
 		mSysStatInc(&memstats.heap_idle, asize)
 
-		// Recalculate nBase
+		// Recalculate nBase.
+		// We know this won't overflow, because sysAlloc returned
+		// a valid region starting at h.curArena.base which is at
+		// least ask bytes in size.
 		nBase = alignUp(h.curArena.base+ask, physPageSize)
 	}