diff --git a/src/libvirt-domain.c b/src/libvirt-domain.c
index 94e5672ed8..83abad251e 100644
--- a/src/libvirt-domain.c
+++ b/src/libvirt-domain.c
@@ -6389,8 +6389,9 @@ virDomainBlockPeek(virDomainPtr dom,
  * to the next alignment boundary.
  *
  * If @flag contains VIR_DOMAIN_BLOCK_RESIZE_CAPACITY (since 10.0.0) the
- * hypervisor will resize the guest block device to fully fill the source,
- * ignoring @size. This is possible only for image formats with no metadata
+ * hypervisor will resize the guest block device to fully fill the source.
+ * @size must be either set to zero, or to the exact size of the block
+ * device source. This is possible only for image formats with no metadata
  * ('raw') and for source devices with limited capacity such as block devices.
  *
  * The @disk parameter is either an unambiguous source name of the
diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
index ee7774f115..448e6b1591 100644
--- a/src/qemu/qemu_driver.c
+++ b/src/qemu/qemu_driver.c
@@ -9324,7 +9324,14 @@ qemuDomainBlockResize(virDomainPtr dom,
             goto endjob;
         }
 
-        size = disk->src->physical;
+        if (size == 0) {
+            size = disk->src->physical;
+        } else if (size != disk->src->physical) {
+            virReportError(VIR_ERR_INVALID_ARG,
+                           _("Requested resize to '%1$llu' but device size is '%2$llu'"),
+                           size, disk->src->physical);
+            goto endjob;
+        }
     }
 
     /* qcow2 and qed must be sized on 512 byte blocks/sectors,