diff --git a/README.md b/README.md
index 78f01c9..8049ced 100644
--- a/README.md
+++ b/README.md
@@ -22,3 +22,5 @@ This repro includes a `BASIS.SYSTEM` implementation that takes the passed file a
     * ...
 
 And then Bitsy Bye will be able to launch them.
+
+If invoked on a file type other than $F1...$F4 it will QUIT back to ProDOS.
diff --git a/basis.system.s b/basis.system.s
index e045904..df275a2 100644
--- a/basis.system.s
+++ b/basis.system.s
@@ -69,6 +69,20 @@ reloc_start:
 launcher_filename:
         PASCAL_STRING "DOS3.3.LAUNCHER"
 
+.proc get_file_info_params
+param_count:    .byte   $A
+pathname:       .addr   filename_buffer
+access:         .byte   0
+file_type:      .byte   0
+aux_type:       .word   0
+storage_type:   .byte   0
+blocks_used:    .word   0
+mod_date:       .word   0
+mod_time:       .word   0
+create_date:    .word   0
+create_time:    .word   0
+.endproc
+
 .proc open_params
 param_count:    .byte   3
 path:           .addr   launcher_filename
@@ -97,6 +111,16 @@ pathname:       .addr   launcher_prefix
 
 newstart:
 
+;;; Check file type is in $F1...$F4
+
+        MLI_CALL GET_FILE_INFO, get_file_info_params
+        bcs     quit
+        lda     get_file_info_params::file_type
+        cmp     #$F1
+        bcc     quit
+        cmp     #$F4+1
+        bcs     quit
+
 ;;; Find DOS3.3.LAUNCHER
 
 check_for_launcher: