diff --git a/addon/-private/system/relationships/state/has-many.js b/addon/-private/system/relationships/state/has-many.js
index ddfefc219a3..f961954d960 100644
--- a/addon/-private/system/relationships/state/has-many.js
+++ b/addon/-private/system/relationships/state/has-many.js
@@ -89,6 +89,8 @@ export default class ManyRelationship extends Relationship {
     if (this.members.has(record)) {
       return;
     }
+
+    assertPolymorphicType(this.record, this.relationshipMeta, record);
     super.addRecord(record, idx);
     // make lazy later
     this.manyArray.internalAddInternalModels([record], idx);
@@ -130,8 +132,6 @@ export default class ManyRelationship extends Relationship {
   }
 
   notifyRecordRelationshipAdded(record, idx) {
-    assertPolymorphicType(this.record, this.relationshipMeta, record);
-
     this.record.notifyHasManyAdded(this.key, record, idx);
   }
 
diff --git a/addon/-private/system/relationships/state/relationship.js b/addon/-private/system/relationships/state/relationship.js
index d2e07be98cb..582e2fa799a 100644
--- a/addon/-private/system/relationships/state/relationship.js
+++ b/addon/-private/system/relationships/state/relationship.js
@@ -95,12 +95,7 @@ export default class Relationship {
 
     allMembers.forEach(inverseInternalModel => {
       let relationship = inverseInternalModel._relationships.get(this.inverseKey);
-      // TODO: there is always a relationship in this case; this guard exists
-      // because there are tests that fail in teardown after putting things in
-      // invalid state
-      if (relationship) {
-        relationship.inverseDidDematerialize();
-      }
+      relationship.inverseDidDematerialize();
     });
   }