diff --git a/Spine/Resource.swift b/Spine/Resource.swift
index 3213ff7c..78557c60 100644
--- a/Spine/Resource.swift
+++ b/Spine/Resource.swift
@@ -162,7 +162,7 @@ open class Resource: NSObject, NSCoding {
 
 extension Resource {
 	override open var description: String {
-		return "\(resourceType)(\(id), \(url))"
+		return "\(resourceType)(\(String(describing: id)), \(String(describing: url)))"
 	}
 	
 	override open var debugDescription: String {
diff --git a/Spine/SerializeOperation.swift b/Spine/SerializeOperation.swift
index 8d73235c..087969b2 100644
--- a/Spine/SerializeOperation.swift
+++ b/Spine/SerializeOperation.swift
@@ -50,7 +50,7 @@ class SerializeOperation: Operation {
 	// MARK: Serializing
 	
 	fileprivate func serializeResource(_ resource: Resource) -> [String: Any] {
-		Spine.logDebug(.serializing, "Serializing resource \(resource) of type '\(resource.resourceType)' with id '\(resource.id)'")
+		Spine.logDebug(.serializing, "Serializing resource \(resource) of type '\(resource.resourceType)' with id '\(String(describing: resource.id))'")
 		
 		var serializedData: [String: Any] = [:]
 		
diff --git a/Spine/ValueFormatter.swift b/Spine/ValueFormatter.swift
index e53b9ca0..a18fcf57 100644
--- a/Spine/ValueFormatter.swift
+++ b/Spine/ValueFormatter.swift
@@ -133,13 +133,16 @@ struct ValueFormatterRegistry {
 /// If a baseURL has been configured in the URLAttribute, and the given String is not an absolute URL,
 /// it will return an absolute URL, relative to the baseURL.
 private struct URLValueFormatter: ValueFormatter {
-	func unformatValue(_ value: String, forAttribute attribute: URLAttribute) -> URL {
-		return URL(string: value, relativeTo: attribute.baseURL as URL?)!
-	}
-	
-	func formatValue(_ value: URL, forAttribute attribute: URLAttribute) -> String {
-		return value.absoluteString
-	}
+    func unformatValue(_ value: String?, forAttribute attribute: URLAttribute) -> URL? {
+        guard let value = value else {
+            return nil
+        }
+        return URL(string: value, relativeTo: attribute.baseURL)
+    }
+    
+    func formatValue(_ value: URL?, forAttribute attribute: URLAttribute) -> String? {
+        return value?.absoluteString
+    }
 }
 
 /// DateValueFormatter is a value formatter that transforms between NSDate and String, and vice versa.