Index: src/OFObject.m
==================================================================
--- src/OFObject.m
+++ src/OFObject.m
@@ -265,10 +265,14 @@
 +  (IMP)replaceClassMethod: (SEL)selector
   withClassMethodFromClass: (Class)class;
 {
 	IMP newimp;
 
+	if (![class isSubclassOfClass: self])
+		@throw [OFInvalidArgumentException newWithClass: self
+						       selector: _cmd];
+
 #if defined(OF_OBJFW_RUNTIME)
 	newimp = objc_get_class_method(class, selector);
 #elif defined(OF_APPLE_RUNTIME)
 	newimp = method_getImplementation(class_getClassMethod(class,
 	    selector));
@@ -316,10 +320,14 @@
 
 +  (IMP)replaceInstanceMethod: (SEL)selector
   withInstanceMethodFromClass: (Class)class;
 {
 	IMP newimp;
+
+	if (![class isSubclassOfClass: self])
+		@throw [OFInvalidArgumentException newWithClass: self
+						       selector: _cmd];
 
 #if defined(OF_OBJFW_RUNTIME)
 	newimp = objc_get_instance_method(class, selector);
 #elif defined(OF_APPLE_RUNTIME)
 	newimp = class_getMethodImplementation(class, selector);