Update to Drupal 8.1.5. For more information, see https://www.drupal.org/project/drupal/releases/8.1.5

core/modules/taxonomy/migration_templates/d6_taxonomy_term.yml

@@ -13,10 +13,19 @@ process:
   name: name
   description: description
   weight: weight
+  # Only attempt to stub real (non-zero) parents.
+  parent_id:
+    -
+      plugin: skip_on_empty
+      method: process
+      source: parent
+    -
+      plugin: migration
+      migration: d6_taxonomy_term
   parent:
-    plugin: migration
-    migration: d6_taxonomy_term
-    source: parent
+    plugin: default_value
+    default_value: 0
+    source: '@parent_id'
   changed: timestamp
 destination:
   plugin: entity:taxonomy_term

core/modules/taxonomy/migration_templates/d7_taxonomy_term.yml

@@ -11,12 +11,22 @@ process:
     migration: d7_taxonomy_vocabulary
     source: vid
   name: name
-  description: description
+  'description/value': description
+  'description/format': format
   weight: weight
+  # Only attempt to stub real (non-zero) parents.
+  parent_id:
+    -
+      plugin: skip_on_empty
+      method: process
+      source: parent
+    -
+      plugin: migration
+      migration: d7_taxonomy_term
   parent:
-    plugin: migration
-    migration: d7_taxonomy_term
-    source: parent
+    plugin: default_value
+    default_value: 0
+    source: '@parent_id'
   changed: timestamp
 destination:
   plugin: entity:taxonomy_term