diff --git a/AK/NonnullRefPtr.h b/AK/NonnullRefPtr.h
index 53ab7aeccd..a055ccc681 100644
--- a/AK/NonnullRefPtr.h
+++ b/AK/NonnullRefPtr.h
@@ -18,6 +18,9 @@
 
 namespace AK {
 
+template<typename T>
+class OwnPtr;
+
 template<typename T>
 inline void ref_if_not_null(T* ptr)
 {
@@ -93,6 +96,11 @@ public:
 #endif
     }
 
+    template<typename U>
+    NonnullRefPtr(const OwnPtr<U>&) = delete;
+    template<typename U>
+    NonnullRefPtr& operator=(const OwnPtr<U>&) = delete;
+
     NonnullRefPtr& operator=(const NonnullRefPtr& other)
     {
         if (m_ptr != other.m_ptr) {
diff --git a/AK/OwnPtr.h b/AK/OwnPtr.h
index a10cc5bdb0..ba93fd63b3 100644
--- a/AK/OwnPtr.h
+++ b/AK/OwnPtr.h
@@ -7,6 +7,13 @@
 
 namespace AK {
 
+template<typename T>
+class RefPtr;
+template<typename T>
+class NonnullRefPtr;
+template<typename T>
+class WeakPtr;
+
 template<typename T>
 class OwnPtr {
 public:
@@ -36,6 +43,19 @@ public:
 #endif
     }
 
+    template<typename U>
+    OwnPtr(const RefPtr<U>&) = delete;
+    template<typename U>
+    OwnPtr(const NonnullRefPtr<U>&) = delete;
+    template<typename U>
+    OwnPtr(const WeakPtr<U>&) = delete;
+    template<typename U>
+    OwnPtr& operator=(const RefPtr<U>&) = delete;
+    template<typename U>
+    OwnPtr& operator=(const NonnullRefPtr<U>&) = delete;
+    template<typename U>
+    OwnPtr& operator=(const WeakPtr<U>&) = delete;
+
     OwnPtr& operator=(OwnPtr&& other)
     {
         if (this != &other) {
diff --git a/AK/RefPtr.h b/AK/RefPtr.h
index 349fb1de1c..99ebca89b9 100644
--- a/AK/RefPtr.h
+++ b/AK/RefPtr.h
@@ -6,6 +6,9 @@
 
 namespace AK {
 
+template<typename T>
+class OwnPtr;
+
 template<typename T>
 class RefPtr {
 public:
@@ -86,6 +89,11 @@ public:
     }
     RefPtr(std::nullptr_t) {}
 
+    template<typename U>
+    RefPtr(const OwnPtr<U>&) = delete;
+    template<typename U>
+    RefPtr& operator=(const OwnPtr<U>&) = delete;
+
     RefPtr& operator=(RefPtr&& other)
     {
         if (this != &other) {