diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 7b7b39630..aedb0d533 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -7,7 +7,7 @@ Since last release
 
 **Added:**
 * Added package parameter to storage (#603, #612, #616)
-* Added package parameter to source (#613, #617)
+* Added package parameter to source (#613, #617, #621)
 * Added default keep packaging to reactor (#618, #619)
 
 **Changed:**
diff --git a/src/source.cc b/src/source.cc
index 99b578645..751aaa763 100644
--- a/src/source.cc
+++ b/src/source.cc
@@ -102,8 +102,9 @@ std::set<cyclus::BidPortfolio<cyclus::Material>::Ptr> Source::GetMatlBids(
     double qty = std::min(target->quantity(), max_qty);
 
     // calculate packaging
-    double bid_qty = context()->GetPackage(package)->GetFillMass(qty);
-    int n_full_bids = static_cast<int>(std::floor(qty / bid_qty));
+    std::pair<double, int> fill = context()->GetPackage(package)->GetFillMass(qty);
+    double bid_qty = fill.first;
+    int n_full_bids = fill.second;
     Package::ExceedsSplitLimits(n_full_bids);
 
     std::vector<double> bids;