diff --git a/src/main/java/gov/usgs/earthquake/nshmp/calc/DisaggDataset.java b/src/main/java/gov/usgs/earthquake/nshmp/calc/DisaggDataset.java
index 46ed521df15328e9d2ac625e6eea4339276efc40..6754d0204170a468118ff89f09d189538b96b658 100644
--- a/src/main/java/gov/usgs/earthquake/nshmp/calc/DisaggDataset.java
+++ b/src/main/java/gov/usgs/earthquake/nshmp/calc/DisaggDataset.java
@@ -491,6 +491,9 @@ final class DisaggDataset {
           case FAULT_SYSTEM:
             putOrAddSystem((SystemContributor) disaggContributor);
             break;
+          case INTERFACE_SYSTEM:
+            putOrAddSystem((SystemContributor) disaggContributor);
+            break;
           default:
             putOrAddSource((SourceContributor) disaggContributor);
         }
diff --git a/src/main/java/gov/usgs/earthquake/nshmp/calc/Disaggregator.java b/src/main/java/gov/usgs/earthquake/nshmp/calc/Disaggregator.java
index 890106785db1f3056302bf8fb36924b9900306fb..4487a2ffa73b0290a1ee1b2102d3bf529f5583ba 100644
--- a/src/main/java/gov/usgs/earthquake/nshmp/calc/Disaggregator.java
+++ b/src/main/java/gov/usgs/earthquake/nshmp/calc/Disaggregator.java
@@ -91,6 +91,8 @@ final class Disaggregator {
         return processClusterSources();
       case FAULT_SYSTEM:
         return processSystemSources();
+      case INTERFACE_SYSTEM:
+        return processSystemSources();
       default:
         return processSources();
     }