Index: src/OFIntrospection.m
==================================================================
--- src/OFIntrospection.m
+++ src/OFIntrospection.m
@@ -385,12 +385,15 @@
 - (instancetype)of_initWithIvar: (Ivar)ivar
 {
 	self = [super init];
 
 	@try {
-		_name = [[OFString alloc]
-		    initWithUTF8String: ivar_getName(ivar)];
+		const char *name = ivar_getName(ivar);
+
+		if (name != NULL)
+			_name = [[OFString alloc] initWithUTF8String:
+			    (const char *_Nonnull)ivar_getName(ivar)];
 		_typeEncoding = ivar_getTypeEncoding(ivar);
 		_offset = ivar_getOffset(ivar);
 	} @catch (id e) {
 		[self release];
 		@throw e;

Index: src/OFObject.h
==================================================================
--- src/OFObject.h
+++ src/OFObject.h
@@ -208,11 +208,11 @@
 /*!
  * @brief Returns the superclass of the object.
  *
  * @return The superclass of the object
  */
-- (Class)superclass;
+- (nullable Class)superclass;
 
 /*!
  * @brief Returns a boolean whether the object of the specified kind.
  *
  * @param class_ The class whose kind is checked
@@ -505,11 +505,11 @@
 /*!
  * @brief Returns the superclass of the class.
  *
  * @return The superclass of the class
  */
-+ (Class)superclass;
++ (nullable Class)superclass;
 
 /*!
  * @brief Checks whether instances of the class respond to a given selector.
  *
  * @param selector The selector which should be checked for respondence

Index: src/OFObject.m
==================================================================
--- src/OFObject.m
+++ src/OFObject.m
@@ -352,20 +352,29 @@
 }
 
 + (IMP)replaceClassMethod: (SEL)selector
       withMethodFromClass: (Class)class
 {
+	IMP method = [class methodForSelector: selector];
+
+	if (method == NULL)
+		@throw [OFInvalidArgumentException exception];
+
 	return class_replaceMethod(object_getClass(self), selector,
-	    [class methodForSelector: selector],
+	    (IMP _Nonnull)method,
 	    typeEncodingForSelector(object_getClass(class), selector));
 }
 
 + (IMP)replaceInstanceMethod: (SEL)selector
 	 withMethodFromClass: (Class)class
 {
-	return class_replaceMethod(self, selector,
-	    [class instanceMethodForSelector: selector],
+	IMP method = [class instanceMethodForSelector: selector];
+
+	if (method == NULL)
+		@throw [OFInvalidArgumentException exception];
+
+	return class_replaceMethod(self, selector, (IMP _Nonnull)method,
 	    typeEncodingForSelector(class, selector));
 }
 
 + (void)inheritMethodsFromClass: (Class)class
 {
@@ -454,11 +463,11 @@
 	} @finally {
 		free(methodList);
 	}
 #endif
 
-	[self inheritMethodsFromClass: [class superclass]];
+	[self inheritMethodsFromClass: superclass];
 }
 
 + (BOOL)resolveClassMethod: (SEL)selector
 {
 	return NO;
@@ -474,11 +483,11 @@
 	return self;
 }
 
 - (Class)class
 {
-	return object_getClass(self);
+	return (Class _Nonnull)object_getClass(self);
 }
 
 - (Class)superclass
 {
 	return class_getSuperclass(object_getClass(self));

Index: src/instance.m
==================================================================
--- src/instance.m
+++ src/instance.m
@@ -55,11 +55,11 @@
 	id obj = (id)bytes;
 
 	if (cls == Nil || bytes == NULL)
 		return nil;
 
-	object_setClass(obj, cls);
+	object_setClass(obj, (Class _Nonnull)cls);
 
 	if (!callConstructors(cls, obj))
 		return nil;
 
 	return obj;