diff --git a/uppsrc/TCore/CalcNode.cpp b/uppsrc/TCore/CalcNode.cpp
index 989ef1b19..45ab11e02 100644
--- a/uppsrc/TCore/CalcNode.cpp
+++ b/uppsrc/TCore/CalcNode.cpp
@@ -625,10 +625,10 @@ Time CalcContext::EvaluateTime(String expr)
 	return !!node ? node->CalcTime(*this) : Time(Null);
 }
 
-bool CalcContext::EvaluateBool(String expr)
+bool CalcContext::EvaluateBool(String expr, bool null_value)
 {
 	CalcNodePtr node = CalcParser().ScanVoid(expr);
-	return !!node && node->CalcBool(*this);
+	return !!node ? node->CalcBool(*this) : null_value;
 }
 
 String CalcContext::OptimizeConstant(String expr)
diff --git a/uppsrc/TCore/CalcNode.h b/uppsrc/TCore/CalcNode.h
index 34b344e86..f7e53b23b 100644
--- a/uppsrc/TCore/CalcNode.h
+++ b/uppsrc/TCore/CalcNode.h
@@ -350,7 +350,7 @@ public:
 	int                 EvaluateInt(String expr);
 	Date                EvaluateDate(String expr);
 	Time                EvaluateTime(String expr);
-	bool                EvaluateBool(String expr);
+	bool                EvaluateBool(String expr, bool null_value = false);
 
 	String              OptimizeConstant(String expr);
 	CalcNodePtr         OptimizeConstant(CalcNodePtr src);