diff --git a/js-assembler/package.rkt b/js-assembler/package.rkt
index 7243d4a..c9bf6d7 100644
--- a/js-assembler/package.rkt
+++ b/js-assembler/package.rkt
@@ -11,6 +11,7 @@
          "../parser/parse-bytecode.rkt"
          "../resource/structs.rkt"
 	 "../promise.rkt"
+         "../get-module-bytecode.rkt"
          (prefix-in hash-cache: "hash-cache.rkt")
          racket/match
          racket/list
@@ -19,6 +20,8 @@
          racket/path
          racket/string
          racket/port
+         syntax/modread
+         syntax/kerncase
          (prefix-in query: "../lang/js/query.rkt")
          (prefix-in resource-query: "../resource/query.rkt")
          (prefix-in runtime: "get-runtime.rkt")
@@ -92,6 +95,61 @@
 
 
 
+;; check-valid-source: Source -> void
+;; Check to see if the file, if a module, is a valid module file.
+(define (check-valid-source src)
+  (cond
+   [(StatementsSource? src)
+    (void)]
+   [(MainModuleSource? src)
+    (check-valid-module-source (MainModuleSource-path src))]
+   [(ModuleSource? src)
+    (check-valid-module-source (ModuleSource-path src))]
+   [(SexpSource? src)
+    (void)]
+   [(UninterpretedSource? src)
+    (void)]))
+
+(define (check-valid-module-source module-source-path)
+  ;; Check that the file exists.
+  (unless (file-exists? module-source-path)
+    (printf "Can't read a Racket module from ~e.  The file does not appear to exist.\n"
+            module-source-path)
+    (error 'check-valid-module-source))
+
+  ;; Check that it looks like a module.
+  (define stx
+    (with-handlers ([exn:fail?
+                     (lambda (exn)
+                       ;; We can't even get the bytecode for the file.
+                       ;; Fail immediately.
+                       (printf "Can't read a Racket module from ~e.  The file may be ill-formed.\n"
+                               module-source-path)
+                       (printf "\nFor reference, the error message produced when trying to read ~e is:\n\n" module-source-path)
+                       (printf "~a\n" (exn-message exn))
+                       (error 'check-valid-module-source))])
+      (parameterize ([read-accept-reader #t])
+        (call-with-input-file* module-source-path
+                               (lambda (ip)
+                                 (read-syntax #f ip))))))
+
+  (define language-stx
+    (kernel-syntax-case stx #t
+      [(module name language body ...)
+       #'language]
+      [else
+       (printf "Can't read a Racket module from ~e.\nThe file exists, but does not appear to be a Racket module.\n"
+               module-source-path)
+       (error 'check-valid-module-source)]))
+
+  ;; Check that the module is written in a language that we allow.
+  (displayln language-stx)
+  (void))
+
+   
+
+
+
 ;; source-is-javascript-module?: Source -> boolean
 ;; Returns true if the source looks like a Javascript-implemented module.
 (define (source-is-javascript-module? src)
@@ -256,6 +314,9 @@ M.modules[~s] =
   ;; Translate all JavaScript-implemented sources into uninterpreted sources;
   ;; we'll leave its interpretation to on-visit-src.
   (define (wrap-source src)
+    (log-debug "Checking valid source")
+    (check-valid-source src)
+    
     (log-debug "Checking if the source has a JavaScript implementation")
     (cond
       [(source-is-javascript-module? src)