diff --git a/packages/compartment-mapper/src/bundle.js b/packages/compartment-mapper/src/bundle.js
index 68adc32d21..c4b540e78d 100644
--- a/packages/compartment-mapper/src/bundle.js
+++ b/packages/compartment-mapper/src/bundle.js
@@ -113,8 +113,14 @@ const sortedModules = (
     const source = compartmentSources[compartmentName][moduleSpecifier];
     if (source !== undefined) {
       const { record, parser, deferredError, bytes } = source;
-      assert(parser !== undefined);
-      assert(bytes !== undefined);
+      assert(
+        bytes !== undefined,
+        `No bytes for ${moduleSpecifier} in ${compartmentName}`,
+      );
+      assert(
+        parser !== undefined,
+        `No parser for ${moduleSpecifier} in ${compartmentName}`,
+      );
       if (deferredError) {
         throw Error(
           `Cannot bundle: encountered deferredError ${deferredError}`,