diff --git a/lib/src/main/java/com/otaliastudios/transcoder/internal/thumbnails/DefaultThumbnailsEngine.kt b/lib/src/main/java/com/otaliastudios/transcoder/internal/thumbnails/DefaultThumbnailsEngine.kt
index bead87b0..c2ae7066 100644
--- a/lib/src/main/java/com/otaliastudios/transcoder/internal/thumbnails/DefaultThumbnailsEngine.kt
+++ b/lib/src/main/java/com/otaliastudios/transcoder/internal/thumbnails/DefaultThumbnailsEngine.kt
@@ -75,7 +75,7 @@ class DefaultThumbnailsEngine(
     ) {
         var actualLocalizedUs: Long = localizedUs
         override fun toString(): String {
-            return positionUs.toString()
+            return request.sourceId() + ":" + positionUs.toString()
         }
     }
 
@@ -91,6 +91,8 @@ class DefaultThumbnailsEngine(
 
         override fun getSeekThreshold() = source.seekThreshold
 
+        override fun mediaId() = source.mediaId()
+
         override fun isDrained(): Boolean {
             if (source.isDrained) {
                 source.seekTo(stubs.firstOrNull()?.positionUs ?: -1)
diff --git a/lib/src/main/java/com/otaliastudios/transcoder/source/UriDataSource.java b/lib/src/main/java/com/otaliastudios/transcoder/source/UriDataSource.java
index 82562773..65c8fc7d 100644
--- a/lib/src/main/java/com/otaliastudios/transcoder/source/UriDataSource.java
+++ b/lib/src/main/java/com/otaliastudios/transcoder/source/UriDataSource.java
@@ -32,4 +32,9 @@ protected void initializeExtractor(@NonNull MediaExtractor extractor) throws IOE
     protected void initializeRetriever(@NonNull MediaMetadataRetriever retriever) {
         retriever.setDataSource(context, uri);
     }
+
+    @Override
+    public String mediaId() {
+        return uri.toString();
+    }
 }
diff --git a/lib/src/main/java/com/otaliastudios/transcoder/thumbnail/ThumbnailRequest.kt b/lib/src/main/java/com/otaliastudios/transcoder/thumbnail/ThumbnailRequest.kt
index 8724b7ba..89a669ff 100644
--- a/lib/src/main/java/com/otaliastudios/transcoder/thumbnail/ThumbnailRequest.kt
+++ b/lib/src/main/java/com/otaliastudios/transcoder/thumbnail/ThumbnailRequest.kt
@@ -4,6 +4,8 @@ interface ThumbnailRequest {
     fun locate(durationUs: Long): List<Long>
 
     fun threshold(): Long = 0
+
+    fun sourceId() : String = ""
     // Could make it so that if locate() is empty, accept is called for each frame (no seeking).
     // But this only makes sense if accept signature has more information (segment, ...), and
     // it should also have a way to say - we're done, stop transcoding.