diff --git a/Source/library/src/main/java/im/delight/android/ddp/db/memory/InMemoryCollection.java b/Source/library/src/main/java/im/delight/android/ddp/db/memory/InMemoryCollection.java
index 4c88071..d20bfb2 100644
--- a/Source/library/src/main/java/im/delight/android/ddp/db/memory/InMemoryCollection.java
+++ b/Source/library/src/main/java/im/delight/android/ddp/db/memory/InMemoryCollection.java
@@ -16,7 +16,7 @@
  * limitations under the License.
  */
 
-import java.util.LinkedHashMap;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.Map;
 import im.delight.android.ddp.db.Collection;
 import im.delight.android.ddp.db.Document;
@@ -145,7 +145,7 @@ public String toString() {
 	}
 
 	/** Data type for the map backing the collection */
-	protected static class DocumentsMap extends LinkedHashMap<String, InMemoryDocument> {
+	protected static class DocumentsMap extends ConcurrentHashMap<String, InMemoryDocument> {
 
 		public DocumentsMap() {
 			super();
diff --git a/Source/library/src/main/java/im/delight/android/ddp/db/memory/InMemoryDatabase.java b/Source/library/src/main/java/im/delight/android/ddp/db/memory/InMemoryDatabase.java
index 5e26106..876f097 100644
--- a/Source/library/src/main/java/im/delight/android/ddp/db/memory/InMemoryDatabase.java
+++ b/Source/library/src/main/java/im/delight/android/ddp/db/memory/InMemoryDatabase.java
@@ -20,7 +20,7 @@
 import im.delight.android.ddp.db.Collection;
 import im.delight.android.ddp.db.Database;
 import im.delight.android.ddp.Fields;
-import java.util.HashMap;
+import java.util.concurrent.ConcurrentHashMap;
 
 /** Database that is stored in memory */
 public final class InMemoryDatabase implements Database {
@@ -108,6 +108,6 @@ public String toString() {
 	}
 
 	/** Data type for the map backing the database */
-	private static class CollectionsMap extends HashMap<String, InMemoryCollection> { }
+	private static class CollectionsMap extends ConcurrentHashMap<String, InMemoryCollection> { }
 
 }