diff --git a/src/codegen/bundle-modules.ts b/src/codegen/bundle-modules.ts
index 8cb8c59986b8f4..b98828c5e5b8b0 100644
--- a/src/codegen/bundle-modules.ts
+++ b/src/codegen/bundle-modules.ts
@@ -81,6 +81,16 @@ for (let i = 0; i < moduleList.length; i++) {
   try {
     let input = fs.readFileSync(path.join(BASE, moduleList[i]), "utf8");
 
+    // NOTE: internal modules are parsed as functions. They must use ESM to export and require to import.
+    // TODO: Bother @paperdave and have him check for module.exports, and create/return a module object if detected.
+    if (!/\bexport\s+(?:function|class|const|default|{)/.test(input)) {
+      if (input.includes("module.exports")) {
+        throw new Error("Cannot use CommonJS module.exports in ESM modules. Use `export default { ... }` instead.");
+      } else {
+        throw new Error("Internal modules must have an `export default` statement.");
+      }
+    }
+
     const scannedImports = t.scanImports(input);
     for (const imp of scannedImports) {
       if (imp.kind === "import-statement") {
@@ -407,7 +417,7 @@ writeIfNotChanged(
     // In this enum are represented as \`(1 << 9) & id\`
     InternalModuleRegistryFlag = 1 << 9,
 ${moduleList.map((id, n) => `    ${idToEnumName(id)} = ${(1 << 9) | n},`).join("\n")}
-    
+
     // Native modules run through the same system, but with different underlying initializers.
     // They also have bit 10 set to differentiate them from JS builtins.
     NativeModuleFlag = (1 << 10) | (1 << 9),
diff --git a/src/codegen/internal-module-registry-scanner.ts b/src/codegen/internal-module-registry-scanner.ts
index 71458fd2e27c11..019040eed676be 100644
--- a/src/codegen/internal-module-registry-scanner.ts
+++ b/src/codegen/internal-module-registry-scanner.ts
@@ -74,7 +74,7 @@ export function createInternalModuleRegistry(basedir: string) {
       const found = moduleList.indexOf(path.relative(basedir, relativeMatch).replaceAll("\\", "/"));
       if (found === -1) {
         throw new Error(
-          `Builtin Bundler: "${specifier}" cannot be imported here because it doesn't get a module ID. Only files in "src/js" besides "src/js/builtins" can be used here. Note that the 'node:' or 'bun:' prefix is required here. `,
+          `Builtin Bundler: "${specifier}" cannot be imported from "${from}" because it doesn't get a module ID. Only files in "src/js" besides "src/js/builtins" can be used here. Note that the 'node:' or 'bun:' prefix is required here. `,
         );
       }
       return codegenRequireId(`${found}/*${path.relative(basedir, relativeMatch)}*/`);