diff --git a/larky/src/main/java/com/verygood/security/larky/modules/JsonModule.java b/larky/src/main/java/com/verygood/security/larky/modules/JsonModule.java
index 23ac892fc..86480afd7 100644
--- a/larky/src/main/java/com/verygood/security/larky/modules/JsonModule.java
+++ b/larky/src/main/java/com/verygood/security/larky/modules/JsonModule.java
@@ -31,6 +31,7 @@
 import net.starlark.java.eval.StarlarkThread;
 import net.starlark.java.eval.StarlarkValue;
 import net.starlark.java.eval.Structure;
+import net.starlark.java.eval.Tuple;
 import net.starlark.java.lib.json.Json;
 
 // Tests at //src/test/java/net/starlark/java/eval:testdata/json.star
@@ -223,8 +224,8 @@ private void encode(Object x) throws EvalException {
         return;
       }
 
-      // e.g. tuple, list
-      if (x instanceof StarlarkIterable) {
+      // only real tuple, list JSON arrays
+      if (x instanceof StarlarkList || x instanceof Tuple) {
         out.append('[');
         String sep = "";
         int i = 0;
@@ -242,6 +243,11 @@ private void encode(Object x) throws EvalException {
         return;
       }
 
+      //StarlarkIterable not list, tuple
+      if (x instanceof StarlarkIterable) {
+        throw Starlark.errorf("Object of type %s is not JSON serializable", Starlark.type(x));
+      }
+
       // e.g. struct
       if (x instanceof Structure) {
         Structure obj = (Structure) x;