diff --git a/Source/cmSystemTools.cxx b/Source/cmSystemTools.cxx
index 557a80d46..1494a3435 100644
--- a/Source/cmSystemTools.cxx
+++ b/Source/cmSystemTools.cxx
@@ -819,7 +819,7 @@ bool cmSystemTools::IsOff(const char* val)
     *c = toupper(*c);
     }
   return (v == "OFF" || v == "0" || v == "NO" || v == "FALSE" || 
-	  v == "N" || v == "NOTFOUND");
+	  v == "N" || v == "NOTFOUND" || v == "IGNORE");
 }
 
 
diff --git a/Source/cmSystemTools.h b/Source/cmSystemTools.h
index 75c53d399..08b6ee164 100644
--- a/Source/cmSystemTools.h
+++ b/Source/cmSystemTools.h
@@ -196,7 +196,7 @@ public:
    * not the same as !IsOn(...) because there are a number of
    * ambiguous values such as "/usr/local/bin" a path will result in
    * IsON and IsOff both returning false. Note that the special path
-   * NOTFOUND will cause IsOff to return true. 
+   * NOTFOUND or IGNORE will cause IsOff to return true. 
    */
   static bool IsOff(const char* val);