diff --git a/src/main/java/gov/usgs/earthquake/nshmp/model/GridSourceSet.java b/src/main/java/gov/usgs/earthquake/nshmp/model/GridSourceSet.java
index fbd1c592c267b2b8341beb43958f1b421f41fe9e..1e980795066d76e7516669358eb5387874e2765b 100644
--- a/src/main/java/gov/usgs/earthquake/nshmp/model/GridSourceSet.java
+++ b/src/main/java/gov/usgs/earthquake/nshmp/model/GridSourceSet.java
@@ -54,6 +54,7 @@ import gov.usgs.earthquake.nshmp.tree.LogicTree;
  */
 public class GridSourceSet extends AbstractSourceSet<PointSource> {
 
+  final SourceType type;
   final List<Location> locations;
   final List<Mfd> mfds;
   final LogicTree<List<Mfd>> mfdsTree;
@@ -86,6 +87,7 @@ public class GridSourceSet extends AbstractSourceSet<PointSource> {
 
   private GridSourceSet(Builder builder) {
     super(builder);
+    this.type = builder.type;
     this.locations = builder.locations;
     this.mfds = builder.mfds;
     this.mfdsTree = builder.mfdsTree;
@@ -105,8 +107,11 @@ public class GridSourceSet extends AbstractSourceSet<PointSource> {
      */
     // System.out.println(Δm);
     // this.optimizable = (sourceType() != FIXED_STRIKE) && !Double.isNaN(Δm);
-    this.optimizable = (sourceType() != FIXED_STRIKE) && this.magMaster.isPresent();
-
+    // TODO magMaster is always present, no?
+    this.optimizable =
+        (type != SourceType.ZONE) &&
+            (sourceType() != FIXED_STRIKE) &&
+            this.magMaster.isPresent();
     // System.out.println(Arrays.toString(magMaster.orElseThrow()));
 
     double[] depthMags = this.mfds.get(0).data().xValues().toArray();
@@ -136,7 +141,7 @@ public class GridSourceSet extends AbstractSourceSet<PointSource> {
 
   @Override
   public SourceType type() {
-    return GRID;
+    return type;
   }
 
   /**
@@ -252,6 +257,7 @@ public class GridSourceSet extends AbstractSourceSet<PointSource> {
 
     private static final String ID = "GridSourceSet.Builder";
 
+    private SourceType type;
     private List<Location> locations = Lists.newArrayList();
     private List<Mfd> mfds = Lists.newArrayList();
     private LogicTree<List<Mfd>> mfdsTree;
@@ -269,6 +275,11 @@ public class GridSourceSet extends AbstractSourceSet<PointSource> {
     private Map<FocalMech, Double> mechMap;
     private boolean singularMechs = true;
 
+    Builder type(SourceType type) {
+      this.type = type;
+      return this;
+    }
+
     Builder strike(double strike) {
       this.strike = OptionalDouble.of(checkStrike(strike));
       this.sourceType = FIXED_STRIKE;
@@ -411,8 +422,9 @@ public class GridSourceSet extends AbstractSourceSet<PointSource> {
     @Override
     void validateState(String buildId) {
       super.validateState(buildId);
+      checkState(type != null, "%s source type not set", buildId);
       checkState(strike != null, "%s strike not set", buildId);
-      checkState(sourceType != null, "%s source type not set", buildId);
+      checkState(sourceType != null, "%s point source type not set", buildId);
       checkState(!locations.isEmpty(), "%s has no locations", buildId);
       checkState(!mfds.isEmpty(), "%s has no Mfds", buildId);
       checkState(rupScaling != null, "%s has no rupture-scaling relation set", buildId);