diff --git a/lib/model/model.go b/lib/model/model.go
index 3a1971817..70fee0ed4 100644
--- a/lib/model/model.go
+++ b/lib/model/model.go
@@ -1576,7 +1576,7 @@ func (m *model) Request(deviceID protocol.DeviceID, folder, name string, size in
 	}
 
 	if !scanner.Validate(res.data, hash, weakHash) {
-		m.recheckFile(deviceID, folderFs, folder, name, int(offset)/int(size), hash)
+		m.recheckFile(deviceID, folderFs, folder, name, size, offset, hash)
 		l.Debugf("%v REQ(in) failed validating data (%v): %s: %q / %q o=%d s=%d", m, err, deviceID, folder, name, offset, size)
 		return nil, protocol.ErrNoSuchFile
 	}
@@ -1584,7 +1584,7 @@ func (m *model) Request(deviceID protocol.DeviceID, folder, name string, size in
 	return res, nil
 }
 
-func (m *model) recheckFile(deviceID protocol.DeviceID, folderFs fs.Filesystem, folder, name string, blockIndex int, hash []byte) {
+func (m *model) recheckFile(deviceID protocol.DeviceID, folderFs fs.Filesystem, folder, name string, size int32, offset int64, hash []byte) {
 	cf, ok := m.CurrentFolderFile(folder, name)
 	if !ok {
 		l.Debugf("%v recheckFile: %s: %q / %q: no current file", m, deviceID, folder, name)
@@ -1596,6 +1596,7 @@ func (m *model) recheckFile(deviceID protocol.DeviceID, folderFs fs.Filesystem,
 		return
 	}
 
+	blockIndex := int(offset) / cf.BlockSize()
 	if blockIndex >= len(cf.Blocks) {
 		l.Debugf("%v recheckFile: %s: %q / %q i=%d: block index too far", m, deviceID, folder, name, blockIndex)
 		return
diff --git a/lib/model/model_test.go b/lib/model/model_test.go
index 040875061..4d20eed99 100644
--- a/lib/model/model_test.go
+++ b/lib/model/model_test.go
@@ -3423,11 +3423,11 @@ func TestIssue5002(t *testing.T) {
 	if !ok {
 		t.Fatal("test file should exist")
 	}
-	nBlocks := len(file.Blocks)
+	blockSize := int32(file.BlockSize())
 
-	m.recheckFile(protocol.LocalDeviceID, defaultFolderConfig.Filesystem(), "default", "foo", nBlocks-1, []byte{1, 2, 3, 4})
-	m.recheckFile(protocol.LocalDeviceID, defaultFolderConfig.Filesystem(), "default", "foo", nBlocks, []byte{1, 2, 3, 4}) // panic
-	m.recheckFile(protocol.LocalDeviceID, defaultFolderConfig.Filesystem(), "default", "foo", nBlocks+1, []byte{1, 2, 3, 4})
+	m.recheckFile(protocol.LocalDeviceID, defaultFolderConfig.Filesystem(), "default", "foo", blockSize, file.Size-int64(blockSize), []byte{1, 2, 3, 4})
+	m.recheckFile(protocol.LocalDeviceID, defaultFolderConfig.Filesystem(), "default", "foo", blockSize, file.Size, []byte{1, 2, 3, 4}) // panic
+	m.recheckFile(protocol.LocalDeviceID, defaultFolderConfig.Filesystem(), "default", "foo", blockSize, file.Size+int64(blockSize), []byte{1, 2, 3, 4})
 }
 
 func TestParentOfUnignored(t *testing.T) {