diff --git a/block-storage/src/main/java/org/lucares/pdb/map/PersistentMap.java b/block-storage/src/main/java/org/lucares/pdb/map/PersistentMap.java
index 3b5b134..a11ae0f 100644
--- a/block-storage/src/main/java/org/lucares/pdb/map/PersistentMap.java
+++ b/block-storage/src/main/java/org/lucares/pdb/map/PersistentMap.java
@@ -35,8 +35,9 @@ public class PersistentMap {
 		}
 	}
 
-	public void put(final String key, final long value) throws IOException {
-		put(key.getBytes(UTF8), VariableByteEncoder.encode(value));
+	public Long put(final String key, final long value) throws IOException {
+		final byte[] oldValue = put(key.getBytes(UTF8), VariableByteEncoder.encode(value));
+		return oldValue == null ? null : VariableByteEncoder.decodeFirstValue(oldValue);
 	}
 
 	public long getAsLong(final String key) throws IOException {
@@ -44,8 +45,9 @@ public class PersistentMap {
 		return buffer == null ? null : VariableByteEncoder.decodeFirstValue(buffer);
 	}
 
-	public void put(final String key, final String value) throws IOException {
-		put(key.getBytes(UTF8), value.getBytes(UTF8));
+	public String put(final String key, final String value) throws IOException {
+		final byte[] oldValue = put(key.getBytes(UTF8), value.getBytes(UTF8));
+		return oldValue == null ? null : new String(oldValue, UTF8);
 	}
 
 	public String getAsString(final String key) throws IOException {
diff --git a/block-storage/src/test/java/org/lucares/pdb/map/PersistentMapTest.java b/block-storage/src/test/java/org/lucares/pdb/map/PersistentMapTest.java
index 7302b39..7883ac0 100644
--- a/block-storage/src/test/java/org/lucares/pdb/map/PersistentMapTest.java
+++ b/block-storage/src/test/java/org/lucares/pdb/map/PersistentMapTest.java
@@ -26,17 +26,21 @@ public class PersistentMapTest {
 		FileUtils.delete(dataDirectory);
 	}
 
-	public void test() throws Exception {
+	public void testSingleValue() throws Exception {
 		final Path file = dataDirectory.resolve("map.db");
 
 		try (final DiskStorage ds = new DiskStorage(file)) {
 			final PersistentMap map = new PersistentMap(ds);
 
-			final String value1 = "value1";
-			map.put("key1", value1);
-			final String actualValue1 = map.getAsString("key1");
+			final String value = "value1";
+			final String key = "key1";
 
-			Assert.assertEquals(actualValue1, value1);
+			Assert.assertNull(map.getAsString(key));
+
+			Assert.assertNull(map.put(key, value));
+			final String actualValue = map.getAsString(key);
+
+			Assert.assertEquals(actualValue, value);
 		}
 	}
 }