diff --git a/src/sink.cpp b/src/sink.cpp
--- a/src/sink.cpp
+++ b/src/sink.cpp
@@ -55,7 +55,7 @@ void Sink::setNumChannels(unsigned nc, b
 
 void Sink::setSource(const Source* s)
 {
-    Q_ASSERT((source == NULL) != (s == NULL));
+    Q_ASSERT((source == nullptr) != (s == nullptr));
     source = s;
 }
 
@@ -63,3 +63,8 @@ const Source* Sink::connectedSource() co
 {
     return source;
 }
+
+Source* Sink::connectedSource()
+{
+    return const_cast<Source*>(static_cast<const Sink&>(*this).connectedSource());
+}