diff --git a/src/runtime/mbitmap.go b/src/runtime/mbitmap.go
index 146ffbfcb6..4d39c321d8 100644
--- a/src/runtime/mbitmap.go
+++ b/src/runtime/mbitmap.go
@@ -164,7 +164,7 @@ func heapBitsForAddr(addr uintptr) heapBits {
 
 // heapBitsForSpan returns the heapBits for the span base address base.
 func heapBitsForSpan(base uintptr) (hbits heapBits) {
-	if base < mheap_.arena_start || base >= mheap_.arena_end {
+	if base < mheap_.arena_start || base >= mheap_.arena_used {
 		throw("heapBitsForSpan: base out of range")
 	}
 	hbits = heapBitsForAddr(base)