diff --git a/src/package.cc b/src/package.cc
index 99bb38000a..fc6829449b 100644
--- a/src/package.cc
+++ b/src/package.cc
@@ -44,7 +44,7 @@ double Package::GetFillMass(double qty) {
     int num_max_fill = std::ceil(qty / fill_max_);
     if (num_min_fill >= num_max_fill) {
       // all material can fit in package(s)
-      double fill_mass = qty / num_max_fill;
+      fill_mass = qty / num_max_fill;
     } else {
       // some material will remain unpackaged, fill up as many max packages as possible
       fill_mass = fill_max_;
diff --git a/tests/package_tests.cc b/tests/package_tests.cc
index 023be279a5..52c80a0d38 100644
--- a/tests/package_tests.cc
+++ b/tests/package_tests.cc
@@ -1,8 +1,11 @@
 #include <string>
 
 #include <gtest/gtest.h>
+#include "error.h"
 
 #include "package.h"
+#include "test_context.h"
+#include "test_agents/test_facility.h"
 
 using cyclus::Package;
 
@@ -40,3 +43,44 @@ TEST(PackageTests, Create) {
 TEST(PackageTests, UnpackagedID) {
     EXPECT_EQ(1, Package::unpackaged_id());
 }
+
+TEST(PackageTests, InvalidPackage) {
+    // can't create package with name "unpackaged"
+    EXPECT_THROW(Package::Create("unpackaged", 0, 1, "first"), cyclus::ValueError);
+    // can't have negative min/max
+    EXPECT_THROW(Package::Create("foo", -1, 1, "first"), cyclus::ValueError);
+    EXPECT_THROW(Package::Create("foo", 0, -1, "first"), cyclus::ValueError);
+    // can't have min bigger than max
+    EXPECT_THROW(Package::Create("foo", 100, 1, "first"), cyclus::ValueError);
+}
+
+TEST(PackageTests, GetFillMass) {
+    double min = 0.3;
+    double max = 0.9;
+    double tight_min = 0.85;
+
+    Package::Ptr p = Package::Create("foo", min, max, "first");
+    Package::Ptr q = Package::Create("bar", min, max, "equal");
+    Package::Ptr r = Package::Create("bar", tight_min, max, "equal");
+
+    double exp;
+
+    double no_fit = 0.05;
+    EXPECT_EQ(0, p->GetFillMass(no_fit));
+    EXPECT_EQ(0, q->GetFillMass(no_fit));
+
+    double perfect_fit = 0.9;
+    EXPECT_EQ(perfect_fit, p->GetFillMass(perfect_fit));
+    EXPECT_EQ(perfect_fit, q->GetFillMass(perfect_fit));
+
+    double partial_fit = 1;
+    EXPECT_EQ(max, p->GetFillMass(partial_fit));
+    exp = partial_fit / 2;
+    EXPECT_EQ(exp, q->GetFillMass(partial_fit));
+    EXPECT_EQ(max, r->GetFillMass(partial_fit));
+
+    double two_packages = 1.4;
+    EXPECT_EQ(max, p->GetFillMass(two_packages));
+    exp = two_packages / 2;
+    EXPECT_EQ(exp, q->GetFillMass(two_packages));
+}
\ No newline at end of file