diff --git a/Sources/ObjectPublisher.swift b/Sources/ObjectPublisher.swift
index ce2cfde..7412fc1 100644
--- a/Sources/ObjectPublisher.swift
+++ b/Sources/ObjectPublisher.swift
@@ -356,7 +356,7 @@ extension ObjectPublisher where O: NSManagedObject {
     public func value<V: AllowedObjectiveCKeyPathValue>(forKeyPath keyPath: KeyPath<O, V>) -> V! {
 
         let key = String(keyPath: keyPath)
-        return self.snapshot?.dictionaryForValues()[key] as! V?
+        return self.snapshot?.dictionaryForValues()[key] as? V
     }
 }
 
diff --git a/Sources/ObjectSnapshot.swift b/Sources/ObjectSnapshot.swift
index 34f7255..259c536 100644
--- a/Sources/ObjectSnapshot.swift
+++ b/Sources/ObjectSnapshot.swift
@@ -120,11 +120,15 @@ public struct ObjectSnapshot<O: DynamicObject>: ObjectRepresentation, Hashable {
     }
 
 
+    // MARK: FilePrivate
+
+    fileprivate var values: [String: Any]
+
+
     // MARK: Private
 
     private let id: O.ObjectID
     private let context: NSManagedObjectContext
-    private var values: [String: Any]
 
     private var valuesRef: NSDictionary {
 
@@ -153,7 +157,16 @@ extension ObjectSnapshot where O: NSManagedObject {
     public func value<V: AllowedObjectiveCKeyPathValue>(forKeyPath keyPath: KeyPath<O, V>) -> V! {
 
         let key = String(keyPath: keyPath)
-        return self.values[key] as! V?
+        return self.values[key] as? V
+    }
+
+    /**
+     Mutates the value for the property identified by a given key.
+     */
+    public mutating func setValue<V: AllowedObjectiveCKeyPathValue>(_ value: V!, forKeyPath keyPath: KeyPath<O, V>) {
+
+        let key = String(keyPath: keyPath)
+        self.values[key] = value
     }
 }
 
@@ -187,7 +200,7 @@ extension ObjectSnapshot where O: CoreStoreObject {
         get {
 
             let key = String(keyPath: member)
-            return self.values[key] as! V?
+            return self.values[key] as? V
         }
         set {
 
@@ -221,7 +234,7 @@ extension ObjectSnapshot where O: CoreStoreObject {
         get {
 
             let key = String(keyPath: member)
-            return self.values[key] as! V?
+            return self.values[key] as? V
         }
         set {
 
@@ -238,7 +251,7 @@ extension ObjectSnapshot where O: CoreStoreObject {
         get {
 
             let key = String(keyPath: member)
-            guard let id = self.values[key] as! D.ObjectID? else {
+            guard let id = self.values[key] as? D.ObjectID else {
 
                 return nil
             }
diff --git a/Sources/PartialObject.swift b/Sources/PartialObject.swift
index 9d42664..82f8731 100644
--- a/Sources/PartialObject.swift
+++ b/Sources/PartialObject.swift
@@ -191,7 +191,7 @@ public struct PartialObject<O: CoreStoreObject> {
      */
     public func value<V>(for property: (O) -> TransformableContainer<O>.Optional<V>) -> V? {
         
-        return self.rawObject.value(forKey: property(O.meta).keyPath) as! V?
+        return self.rawObject.value(forKey: property(O.meta).keyPath) as? V
     }
     
     /**
@@ -212,7 +212,7 @@ public struct PartialObject<O: CoreStoreObject> {
      */
     public func primitiveValue<V>(for property: (O) -> TransformableContainer<O>.Optional<V>) -> V? {
         
-        return self.rawObject.primitiveValue(forKey: property(O.meta).keyPath) as! V?
+        return self.rawObject.primitiveValue(forKey: property(O.meta).keyPath) as? V
     }
     
     /**
diff --git a/Sources/Transformable.swift b/Sources/Transformable.swift
index f26ed6c..182f0bc 100644
--- a/Sources/Transformable.swift
+++ b/Sources/Transformable.swift
@@ -380,7 +380,7 @@ public enum TransformableContainer<O: CoreStoreObject> {
 
                         return customGetter(PartialObject<O>(object))
                     }
-                    return object.value(forKey: self.keyPath) as! V?
+                    return object.value(forKey: self.keyPath) as? V
                 }
             }
             set {
@@ -489,7 +489,7 @@ public enum TransformableContainer<O: CoreStoreObject> {
                 }
                 customSetter(
                     PartialObject<O>(rawObject),
-                    newValue as! V?
+                    newValue as? V
                 )
             }
         }