diff --git a/pgpainless-core/src/main/java/org/pgpainless/util/MultiMap.java b/pgpainless-core/src/main/java/org/pgpainless/util/MultiMap.java
index c3579e8e..8cdc8e5e 100644
--- a/pgpainless-core/src/main/java/org/pgpainless/util/MultiMap.java
+++ b/pgpainless-core/src/main/java/org/pgpainless/util/MultiMap.java
@@ -89,14 +89,9 @@ public class MultiMap<K, V> {
         vs.remove(v);
     }
 
-    public void putAll(Map<? extends K, ? extends Set<V>> _map) {
-        for (K key : _map.keySet()) {
-            Set<V> vs = this.map.get(key);
-            if (vs == null) {
-                vs = new HashSet<>();
-                this.map.put(key, vs);
-            }
-            vs.addAll(_map.get(key));
+    public void putAll(MultiMap<K, V> other) {
+        for (K key : other.keySet()) {
+            put(key, other.get(key));
         }
     }
 
diff --git a/pgpainless-core/src/test/java/org/pgpainless/MultiMapTest.java b/pgpainless-core/src/test/java/org/pgpainless/MultiMapTest.java
new file mode 100644
index 00000000..1f8722d4
--- /dev/null
+++ b/pgpainless-core/src/test/java/org/pgpainless/MultiMapTest.java
@@ -0,0 +1,132 @@
+/*
+ * Copyright 2018 Paul Schaub.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pgpainless;
+
+import static junit.framework.TestCase.assertEquals;
+import static junit.framework.TestCase.assertFalse;
+import static junit.framework.TestCase.assertNotNull;
+import static junit.framework.TestCase.assertNull;
+import static junit.framework.TestCase.assertTrue;
+import static junit.framework.TestCase.fail;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Set;
+
+import org.junit.Test;
+import org.pgpainless.util.MultiMap;
+
+public class MultiMapTest {
+
+    @Test
+    public void test() {
+        MultiMap<String, String> multiMap = new MultiMap<>();
+        assertTrue(multiMap.isEmpty());
+        assertNull(multiMap.get("alice"));
+        assertFalse(multiMap.containsKey("alice"));
+        assertFalse(multiMap.containsValue("wonderland"));
+        assertEquals(0, multiMap.size());
+
+        multiMap.put("alice", "wonderland");
+        assertFalse(multiMap.isEmpty());
+        assertEquals(1, multiMap.size());
+        assertTrue(multiMap.containsKey("alice"));
+        assertTrue(multiMap.containsValue("wonderland"));
+        assertNotNull(multiMap.get("alice"));
+        assertTrue(multiMap.get("alice").contains("wonderland"));
+
+        multiMap.put("mad", new HashSet<>(Arrays.asList("hatter", "max")));
+        assertEquals(new HashSet<>(Arrays.asList("hatter", "max")), multiMap.get("mad"));
+
+        assertEquals(new HashSet<>(Arrays.asList("mad", "alice")), multiMap.keySet());
+        assertEquals(new HashSet<>(Arrays.asList(
+                Collections.singleton("wonderland"),
+                new HashSet<>(Arrays.asList("hatter", "max")))), new HashSet<>(multiMap.values()));
+
+        Set<Map.Entry<String, Set<String>>> entries = multiMap.entrySet();
+        assertEquals(2, entries.size());
+        for (Map.Entry<String, Set<String>> e : entries) {
+            switch (e.getKey()) {
+                case "alice":
+                    assertEquals(1, e.getValue().size());
+                    assertTrue(e.getValue().contains("wonderland"));
+                    break;
+                case "mad":
+                    assertEquals(2, e.getValue().size());
+                    assertTrue(e.getValue().contains("hatter"));
+                    assertTrue(e.getValue().contains("max"));
+                    break;
+                default:
+                    fail("Illegal key.");
+                    break;
+            }
+        }
+
+        MultiMap<String, String> empty = new MultiMap<>();
+        assertFalse(multiMap.equals(empty));
+        assertEquals(multiMap, multiMap);
+        assertFalse(multiMap.equals(null));
+
+        MultiMap<String, String> map2 = new MultiMap<>();
+        map2.put("alice", "schwarzer");
+        map2.put("dr", "strange");
+
+        multiMap.putAll(map2);
+        assertTrue(multiMap.containsKey("dr"));
+        assertEquals(1, multiMap.get("dr").size());
+        assertTrue(multiMap.get("dr").contains("strange"));
+        assertTrue(multiMap.containsKey("mad"));
+        assertEquals(2, multiMap.get("alice").size());
+        assertTrue(multiMap.get("alice").contains("wonderland"));
+        assertTrue(multiMap.get("alice").contains("schwarzer"));
+
+        multiMap.removeAll("mad");
+        assertFalse(multiMap.containsKey("mad"));
+        assertNull(multiMap.get("mad"));
+
+        multiMap.remove("alice", "wonderland");
+        assertFalse(multiMap.containsValue("wonderland"));
+        assertTrue(multiMap.containsKey("alice"));
+        assertEquals(1, multiMap.get("alice").size());
+        assertTrue(multiMap.get("alice").contains("schwarzer"));
+
+        MultiMap<String, String> copy = new MultiMap<>(multiMap);
+        assertEquals(multiMap, copy);
+
+        copy.removeAll("inexistent");
+        assertEquals(multiMap, copy);
+
+        copy.remove("inexistent", "schwarzer");
+        assertEquals(multiMap, copy);
+
+        assertEquals(multiMap.hashCode(), copy.hashCode());
+
+        copy.clear();
+        assertTrue(copy.isEmpty());
+
+        Map<String, Set<String>> map = new HashMap<>();
+        map.put("key", Collections.singleton("value"));
+
+        MultiMap<String, String> fromMap = new MultiMap<>(map);
+        assertFalse(fromMap.isEmpty());
+        assertEquals(fromMap.get("key"), Collections.singleton("value"));
+
+        assertFalse(fromMap.equals(map));
+    }
+}