diff --git a/src/kits/media/AdapterIO.cpp b/src/kits/media/AdapterIO.cpp
index d18a290e51..2db308524c 100644
--- a/src/kits/media/AdapterIO.cpp
+++ b/src/kits/media/AdapterIO.cpp
@@ -86,7 +86,9 @@ BAdapterIO::Position() const
 status_t
 BAdapterIO::SetSize(off_t size)
 {
-	return B_UNSUPPORTED;
+	AutoWriteLocker(fLock);
+
+	return fBuffer->SetSize(size);
 }