diff --git a/src/model/ZoneHVACPackagedTerminalHeatPump.cpp b/src/model/ZoneHVACPackagedTerminalHeatPump.cpp
index 40cda15284..59bc6aaa74 100644
--- a/src/model/ZoneHVACPackagedTerminalHeatPump.cpp
+++ b/src/model/ZoneHVACPackagedTerminalHeatPump.cpp
@@ -359,6 +359,12 @@ namespace model {
 
     double ZoneHVACPackagedTerminalHeatPump_Impl::dXHeatingCoilSizingRatio() const {
       boost::optional<double> value = getDouble(OS_ZoneHVAC_PackagedTerminalHeatPumpFields::DXHeatingCoilSizingRatio, true);
+      if (!value) {
+        // it is an error if we get here (eg: cloned from an object that had it unset), however we don't want to crash
+        LOG(Error, "Required DX Heating Coil Sizing Ratio not set, defaulting to 1.0");
+        const_cast<ZoneHVACPackagedTerminalHeatPump_Impl*>(this)->setDXHeatingCoilSizingRatio(1.0);  // NOLINT
+        value = getDouble(OS_ZoneHVAC_PackagedTerminalHeatPumpFields::DXHeatingCoilSizingRatio, true);
+      }
       OS_ASSERT(value);
       return value.get();
     }
diff --git a/src/model/test/ZoneHVACPackagedTerminalHeatPump_GTest.cpp b/src/model/test/ZoneHVACPackagedTerminalHeatPump_GTest.cpp
index 0b1dee273d..e5067e4d81 100644
--- a/src/model/test/ZoneHVACPackagedTerminalHeatPump_GTest.cpp
+++ b/src/model/test/ZoneHVACPackagedTerminalHeatPump_GTest.cpp
@@ -40,6 +40,7 @@
 #include "../BoilerHotWater.hpp"
 
 #include <utilities/idd/OS_Curve_Biquadratic_FieldEnums.hxx>
+#include <utilities/idd/OS_ZoneHVAC_PackagedTerminalHeatPump_FieldEnums.hxx>
 
 using namespace openstudio;
 
@@ -308,6 +309,23 @@ TEST_F(ModelFixture, ZoneHVACPackagedTerminalHeatPump_CoilSystemCoolingDXHeatExc
   model::ZoneHVACPackagedTerminalHeatPump pthp(m, availabilitySchedule, fan, heatingCoil, coolingCoil, supplementalHeatingCoil);
 }
 
+TEST_F(ModelFixture, ZoneHVACPackagedTerminalHeatPump_DXHeatingCoilSizingRatio_Unset) {
+  // Test for #5635 - cloning from an object that never had this required field set (eg: an
+  // older template) must not return whatever garbage memory the underlying getter finds
+  model::Model m;
+  model::CoilHeatingDXVariableSpeed heatingCoil(m);
+  model::CoilCoolingDXVariableSpeed coolingCoil(m);
+  model::ScheduleCompact availabilitySchedule(m);
+  model::FanConstantVolume fan(m, availabilitySchedule);
+  model::CoilHeatingElectric supplementalHeatingCoil(m, availabilitySchedule);
+
+  model::ZoneHVACPackagedTerminalHeatPump pthp(m, availabilitySchedule, fan, heatingCoil, coolingCoil, supplementalHeatingCoil);
+  EXPECT_EQ(1.0, pthp.dXHeatingCoilSizingRatio());
+
+  ASSERT_TRUE(pthp.setString(OS_ZoneHVAC_PackagedTerminalHeatPumpFields::DXHeatingCoilSizingRatio, ""));
+  EXPECT_EQ(1.0, pthp.dXHeatingCoilSizingRatio());
+}
+
 TEST_F(ModelFixture, ZoneHVACPackagedTerminalHeatPump_SupplyAirFanOpSch) {
   model::Model m;
   model::CoilHeatingDXVariableSpeed heatingCoil(m);