diff --git a/spring-core/src/main/java/org/springframework/util/CollectionUtils.java b/spring-core/src/main/java/org/springframework/util/CollectionUtils.java
index 1f931b9285f1..d1e9354abda9 100644
--- a/spring-core/src/main/java/org/springframework/util/CollectionUtils.java
+++ b/spring-core/src/main/java/org/springframework/util/CollectionUtils.java
@@ -457,7 +457,6 @@ public static <E> Iterator<E> toIterator(@Nullable Enumeration<E> enumeration) {
 	 * @since 3.1
 	 */
 	public static <K, V> MultiValueMap<K, V> toMultiValueMap(Map<K, List<V>> targetMap) {
-		Assert.notNull(targetMap, "'targetMap' must not be null");
 		return new MultiValueMapAdapter<>(targetMap);
 	}
 
diff --git a/spring-core/src/main/java/org/springframework/util/LinkedMultiValueMap.java b/spring-core/src/main/java/org/springframework/util/LinkedMultiValueMap.java
index 11c7970b33d6..8faf71ea1ce0 100644
--- a/spring-core/src/main/java/org/springframework/util/LinkedMultiValueMap.java
+++ b/spring-core/src/main/java/org/springframework/util/LinkedMultiValueMap.java
@@ -35,7 +35,8 @@
  * @param <K> the key type
  * @param <V> the value element type
  */
-public class LinkedMultiValueMap<K, V> extends MultiValueMapAdapter<K, V> implements Serializable, Cloneable {
+public class LinkedMultiValueMap<K, V> extends MultiValueMapAdapter<K, V>  // new public base class in 5.3
+		implements Serializable, Cloneable {
 
 	private static final long serialVersionUID = 3801124242820219131L;
 
diff --git a/spring-core/src/main/java/org/springframework/util/MultiValueMapAdapter.java b/spring-core/src/main/java/org/springframework/util/MultiValueMapAdapter.java
index c5f0ab33b08b..0d51b56117c8 100644
--- a/spring-core/src/main/java/org/springframework/util/MultiValueMapAdapter.java
+++ b/spring-core/src/main/java/org/springframework/util/MultiValueMapAdapter.java
@@ -30,23 +30,30 @@
  *
  * @author Arjen Poutsma
  * @author Juergen Hoeller
- * @since 3.1
+ * @since 5.3
  * @param <K> the key type
  * @param <V> the value element type
  * @see CollectionUtils#toMultiValueMap
  * @see LinkedMultiValueMap
  */
 @SuppressWarnings("serial")
-class MultiValueMapAdapter<K, V> implements MultiValueMap<K, V>, Serializable {
+public class MultiValueMapAdapter<K, V> implements MultiValueMap<K, V>, Serializable {
 
 	private final Map<K, List<V>> targetMap;
 
 
-	MultiValueMapAdapter(Map<K, List<V>> targetMap) {
+	/**
+	 * Wrap the given target {@link Map} as a {@link MultiValueMap} adapter.
+	 * @param targetMap the plain target {@code Map}
+	 */
+	public MultiValueMapAdapter(Map<K, List<V>> targetMap) {
+		Assert.notNull(targetMap, "'targetMap' must not be null");
 		this.targetMap = targetMap;
 	}
 
 
+	// MultiValueMap implementation
+
 	@Override
 	@Nullable
 	public V getFirst(K key) {
@@ -96,6 +103,9 @@ public Map<K, V> toSingleValueMap() {
 		return singleValueMap;
 	}
 
+
+	// Map implementation
+
 	@Override
 	public int size() {
 		return this.targetMap.size();