diff --git a/src/Magnum/Vk/Memory.cpp b/src/Magnum/Vk/Memory.cpp
index 4356a283d..d4126789f 100644
--- a/src/Magnum/Vk/Memory.cpp
+++ b/src/Magnum/Vk/Memory.cpp
@@ -105,6 +105,7 @@ Containers::Array<char, MemoryMapDeleter> Memory::map(const UnsignedLong offset,
 }
 
 Containers::Array<char, MemoryMapDeleter> Memory::map() {
+    CORRADE_ASSERT(_size, "Vk::Memory::map(): the memory has unknown size, you have to specify it explicitly", {});
     return map(0, _size);
 }
 
@@ -119,6 +120,7 @@ Containers::Array<const char, MemoryMapDeleter> Memory::mapRead(const UnsignedLo
 }
 
 Containers::Array<const char, MemoryMapDeleter> Memory::mapRead() {
+    CORRADE_ASSERT(_size, "Vk::Memory::mapRead(): the memory has unknown size, you have to specify it explicitly", {});
     return mapRead(0, _size);
 }
 
diff --git a/src/Magnum/Vk/Memory.h b/src/Magnum/Vk/Memory.h
index 334a19bcc..483cc2da5 100644
--- a/src/Magnum/Vk/Memory.h
+++ b/src/Magnum/Vk/Memory.h
@@ -329,9 +329,14 @@ class MAGNUM_VK_EXPORT Memory {
          * @param size      Memory size
          * @param flags     Handle flags
          *
-         * The @p handle is expected to be originating from @p device. Unlike
-         * a memory allocated using a constructor, the Vulkan memory is by
-         * default not freed on destruction, use @p flags for different
+         * The @p handle is expected to be originating from @p device. The
+         * @p size parameter will be used to properly size the output array
+         * coming from @ref map(). If a concrete @p size is unknown, use a
+         * zero --- you will then be able to only use the @ref map(UnsignedLong, UnsignedLong)
+         * overload.
+         *
+         * Unlike a memory allocated using a constructor, the Vulkan memory is
+         * by default not freed on destruction, use @p flags for different
          * behavior.
          * @see @ref release()
          */
diff --git a/src/Magnum/Vk/Test/MemoryTest.cpp b/src/Magnum/Vk/Test/MemoryTest.cpp
index 04a93f62f..c85c42841 100644
--- a/src/Magnum/Vk/Test/MemoryTest.cpp
+++ b/src/Magnum/Vk/Test/MemoryTest.cpp
@@ -24,9 +24,11 @@
 */
 
 #include <sstream>
+#include <Corrade/Containers/Array.h>
 #include <Corrade/TestSuite/Tester.h>
 #include <Corrade/Utility/DebugStl.h>
 
+#include "Magnum/Vk/Device.h"
 #include "Magnum/Vk/Memory.h"
 
 namespace Magnum { namespace Vk { namespace Test { namespace {
@@ -47,6 +49,8 @@ struct MemoryTest: TestSuite::Tester {
     void constructNoCreate();
     void constructCopy();
 
+    void mapWrappedUnknownSize();
+
     void debugMemoryFlag();
     void debugMemoryFlags();
 };
@@ -65,6 +69,8 @@ MemoryTest::MemoryTest() {
               &MemoryTest::constructNoCreate,
               &MemoryTest::constructCopy,
 
+              &MemoryTest::mapWrappedUnknownSize,
+
               &MemoryTest::debugMemoryFlag,
               &MemoryTest::debugMemoryFlags});
 }
@@ -160,6 +166,23 @@ void MemoryTest::constructCopy() {
     CORRADE_VERIFY(!std::is_copy_assignable<Memory>{});
 }
 
+void MemoryTest::mapWrappedUnknownSize() {
+    #ifdef CORRADE_NO_ASSERT
+    CORRADE_SKIP("CORRADE_NO_ASSERT defined, can't test assertions");
+    #endif
+
+    Device device{NoCreate};
+    Memory memory = Memory::wrap(device, {}, 0);
+
+    std::ostringstream out;
+    Error redirectError{&out};
+    memory.map();
+    memory.mapRead();
+    CORRADE_COMPARE(out.str(),
+        "Vk::Memory::map(): the memory has unknown size, you have to specify it explicitly\n"
+        "Vk::Memory::mapRead(): the memory has unknown size, you have to specify it explicitly\n");
+}
+
 void MemoryTest::debugMemoryFlag() {
     std::ostringstream out;
     Debug{&out} << MemoryFlag::HostCached << MemoryFlag(0xdeadcafe);