diff --git a/lib/Syntax/Keyword/Assert.xs b/lib/Syntax/Keyword/Assert.xs
index 805ebac..e330aa8 100644
--- a/lib/Syntax/Keyword/Assert.xs
+++ b/lib/Syntax/Keyword/Assert.xs
@@ -213,24 +213,43 @@ ok:
   RETURN;
 }
 
-static int build_assert(pTHX_ OP **out, XSParseKeywordPiece *arg0, void *hookdata)
+static int build_assert(pTHX_ OP **out, XSParseKeywordPiece *args[], size_t nargs, void *hookdata)
 {
-    OP *argop = arg0->op;
+    // assert(EXPR, EXPR)
+    //
+    //  assert($x == 1)
+    //  assert($x == 1, "x is not 1");
+    //  assert($x == 1, sub { "x is not 1: x=$x" });
+    //
+    // first EXPR is the condition, second is the message.
+    // error meeage is optional.
+    // if the condition is false, the message is printed and the program dies.
+    //
+    // TODO implement the error message.
+    //  [ ] First, implement the message as a string.
+    //  [ ] Then, implement the message as a code block.
+    OP *condop = args[0]->op;
+    OP *msgop  = args[2] ? args[2]->op : NULL;
+
     if (assert_enabled) {
-        enum BinopType binoptype = classify_binop(argop->op_type);
+        enum BinopType binoptype = classify_binop(condop->op_type);
         if (binoptype) {
-            argop->op_type = OP_CUSTOM;
-            argop->op_ppaddr = &pp_assertbin;
-            argop->op_private = binoptype;
-            *out = argop;
+            condop->op_type = OP_CUSTOM;
+            condop->op_ppaddr = &pp_assertbin;
+            condop->op_private = binoptype;
+
+            *out = condop;
         }
         else {
-            *out = newUNOP_CUSTOM(&pp_assert, 0, argop);
+            *out = newUNOP_CUSTOM(&pp_assert, 0, condop);
         }
     }
     else {
         // do nothing.
-        op_free(argop);
+        op_free(condop);
+        if (msgop) {
+            op_free(msgop);
+        }
         *out = newOP(OP_NULL, 0);
     }
 
@@ -239,8 +258,15 @@ static int build_assert(pTHX_ OP **out, XSParseKeywordPiece *arg0, void *hookdat
 
 static const struct XSParseKeywordHooks hooks_assert = {
   .permit_hintkey = "Syntax::Keyword::Assert/assert",
-  .piece1 = XPK_TERMEXPR_SCALARCTX,
-  .build1 = &build_assert,
+  .pieces = (const struct XSParseKeywordPieceType[]) {
+    XPK_ARGS(
+      XPK_TERMEXPR_SCALARCTX,
+      XPK_OPTIONAL(XPK_COMMA),
+      XPK_TERMEXPR_SCALARCTX_OPT
+    ),
+    {0}
+  },
+  .build = &build_assert,
 };
 
 MODULE = Syntax::Keyword::Assert    PACKAGE = Syntax::Keyword::Assert