diff --git a/block.c b/block.c
index dd5d5ca4d2..ecc5b442d4 100644
--- a/block.c
+++ b/block.c
@@ -2117,23 +2117,33 @@ typedef struct BdrvCoIsAllocatedData {
  * not implementing the functionality are assumed to not support backing files,
  * hence all their sectors are reported as allocated.
  *
+ * If 'sector_num' is beyond the end of the disk image the return value is 0
+ * and 'pnum' is set to 0.
+ *
  * 'pnum' is set to the number of sectors (including and immediately following
  * the specified sector) that are known to be in the same
  * allocated/unallocated state.
  *
- * 'nb_sectors' is the max value 'pnum' should be set to.
+ * 'nb_sectors' is the max value 'pnum' should be set to.  If nb_sectors goes
+ * beyond the end of the disk image it will be clamped.
  */
 int coroutine_fn bdrv_co_is_allocated(BlockDriverState *bs, int64_t sector_num,
                                       int nb_sectors, int *pnum)
 {
+    int64_t n;
+
+    if (sector_num >= bs->total_sectors) {
+        *pnum = 0;
+        return 0;
+    }
+
+    n = bs->total_sectors - sector_num;
+    if (n < nb_sectors) {
+        nb_sectors = n;
+    }
+
     if (!bs->drv->bdrv_co_is_allocated) {
-        int64_t n;
-        if (sector_num >= bs->total_sectors) {
-            *pnum = 0;
-            return 0;
-        }
-        n = bs->total_sectors - sector_num;
-        *pnum = (n < nb_sectors) ? (n) : (nb_sectors);
+        *pnum = nb_sectors;
         return 1;
     }