diff --git a/dos.js b/dos.js
index 0db332c..23e8d2a 100644
--- a/dos.js
+++ b/dos.js
@@ -21,7 +21,10 @@ function DOS(tty) {
     NO_BUFFERS_AVAILABLE: [12, 'No buffers available'],
     FILE_TYPE_MISMATCH: [13, 'File type mismatch'],
     PROGRAM_TOO_LARGE: [14, 'Program too large'],
-    NOT_DIRECT_COMMAND: [15, 'Not direct command']
+    NOT_DIRECT_COMMAND: [15, 'Not direct command'],
+
+    // Re-used
+    SYNTAX_ERROR: [16, "Syntax error"]
   },
 
       STORAGE_PREFIX = 'vfs/',
@@ -368,7 +371,7 @@ function DOS(tty) {
       activebuffer = null;
       mode = "";
     } else {
-      doserror(DOSErrors.INVALID_OPTION);
+      doserror(DOSErrors.SYNTAX_ERROR);
     }
   }
 
diff --git a/samples/sample.unittests.txt b/samples/sample.unittests.txt
index 506d9c8..83e181a 100644
--- a/samples/sample.unittests.txt
+++ b/samples/sample.unittests.txt
@@ -881,7 +881,7 @@
 18091 T = 1
 18093 S = (T = 2) : GOSUB 1
 
-18100 T$ = "Output Hook Precedence
+18100 T$ = "Output Hook Precedence"
 : A$ = "abc" + CHR$(7) + "xyz"
 : PRINT CHR$(4)"OPEN TESTDATA"
 : PRINT CHR$(4)"WRITE TESTDATA"
@@ -893,6 +893,12 @@
 : PRINT CHR$(4)"CLOSE TESTDATA"
 : S = (A$ = B$) : GOSUB 1
 
+18110 T$ = "Invalid DOS Command"
+: T = 1 : ONERR GOTO 18112
+18111 PRINT CHR$(4);CHR$(5)
+: T = 2
+18112 S = (T=1) AND (PEEK(222)=16) : GOSUB 1
+
 20000 PRINT : PRINT : PRINT "Executed tests: "; TE
 20010 PRINT "Successful tests: "; TS
 20019 IF F THEN INVERSE