diff --git a/lib/json0.js b/lib/json0.js
index 9f538ee..abea8ff 100644
--- a/lib/json0.js
+++ b/lib/json0.js
@@ -70,6 +70,9 @@ json.invertComponent = function(c) {
 
   // handle subtype ops
   if (c.t && subtypes[c.t]) {
+    if (typeof subtypes[c.t].invert !== 'function') {
+      throw new Error("Subtype '" + c.t + "' is not invertible");
+    }
     c_.t = c.t;
     c_.o = subtypes[c.t].invert(c.o);
   }