Update NCIdlCompiler.scala
diff --git a/nlpcraft/src/main/scala/org/apache/nlpcraft/model/intent/compiler/NCIdlCompiler.scala b/nlpcraft/src/main/scala/org/apache/nlpcraft/model/intent/compiler/NCIdlCompiler.scala
index d27133c..7e24d46 100644
--- a/nlpcraft/src/main/scala/org/apache/nlpcraft/model/intent/compiler/NCIdlCompiler.scala
+++ b/nlpcraft/src/main/scala/org/apache/nlpcraft/model/intent/compiler/NCIdlCompiler.scala
@@ -132,7 +132,7 @@
             val varName = ctx.id().getText
 
             if (!vars.contains(varName))
-                throw newSyntaxError(s"Unknown variable: @$varName")(ctx)
+                throw newSyntaxError(s"Undefined variable: @$varName")(ctx)
 
             val instr: SI = (tok: NCToken, stack: S, idlCtx: NCIdlContext) ⇒ stack.push(() ⇒ idlCtx.vars(varName)(tok, idlCtx))