diff --git a/src/main/javascript/core/_primitives.js b/src/main/javascript/core/_primitives.js
index 8ae055a..93207aa 100644
--- a/src/main/javascript/core/_primitives.js
+++ b/src/main/javascript/core/_primitives.js
@@ -52,6 +52,8 @@ var global = this;
     };
 
     var _putSign = function(texts, x, y, z, blockId, meta){
+        if (blockId != 63 && blockId != 68)
+            throw new Error("Invalid Parameter: blockId must be 63 or 68");
         putBlock(x,y,z,blockId,meta);
         var block = _getBlockObject(x,y,z);
         state = block.state;