diff --git a/nasal_codegen.h b/nasal_codegen.h
index 0509ace..24b7902 100644
--- a/nasal_codegen.h
+++ b/nasal_codegen.h
@@ -1,6 +1,31 @@
 #ifndef __NASAL_CODEGEN_H__
 #define __NASAL_CODEGEN_H__
 
+enum op_code
+{
+    op_nop,
+    op_load,
+    op_pushnum,op_pushone,op_pushzero,
+    op_pushnil,
+    op_pushstr,
+    op_newvec,op_newhash,op_newfunc,
+    op_vecappend,op_hashappend,
+    op_unot,op_usub,
+    op_add,op_sub,op_mul,op_div,op_lnk,
+    op_eq,op_neq,op_less,op_leq,op_grt,op_geq,
+    op_jmp,op_jmptrue,op_jmpfalse,
+    op_gccall,op_gccallvec,op_gccallhash,op_gccallfunc,op_builtincall,op_slice,
+    op_memcall,op_memcallvec,op_memcallhash,
+    op_return,
+    op_end
+};
+
+struct opcode
+{
+    unsigned char op;
+    int index;
+};
+
 // unfinished
 // now it can output ast but it is not byte code yet
 // please wait...
diff --git a/nasal_runtime.h b/nasal_runtime.h
index de39234..d22621c 100644
--- a/nasal_runtime.h
+++ b/nasal_runtime.h
@@ -531,7 +531,7 @@ bool nasal_runtime::check_condition(int value_addr)
     if(value_addr<0)
         return false;
     int type=nasal_vm.gc_get(value_addr).get_type();
-    if(type==vm_vector || type==vm_hash || type==vm_function)
+    if(type==vm_nil || type==vm_vector || type==vm_hash || type==vm_function)
         return false;
     else if(type==vm_string)
     {
@@ -541,8 +541,6 @@ bool nasal_runtime::check_condition(int value_addr)
             return false;
         return (number!=0);
     }
-    else if(type==vm_nil)
-        return false;
     else if(type==vm_number)
         return (nasal_vm.gc_get(value_addr).get_number()!=0);
     return false;