diff --git a/src/core/SourceHoldingTank.vala b/src/core/SourceHoldingTank.vala
index 93260e4..4ab52cb 100644
--- a/src/core/SourceHoldingTank.vala
+++ b/src/core/SourceHoldingTank.vala
@@ -164,7 +164,12 @@ public class SourceHoldingTank {
     
     // This is only called by DataSource
     public void internal_notify_altered(DataSource source, Alteration alteration) {
-        assert(tank.contains(source));
+        if (!tank.contains(source)) {
+            debug("SourceHoldingTank.internal_notify_altered called for %s not stored in %s",
+                source.to_string(), to_string());
+            
+            return;
+        }
         
         // see if it should stay put
         if (check_to_keep(source, alteration))
