--- src/libs/zbxserver/expression.c	2013-09-03 14:07:45.000000000 +0300
+++ src/libs/zbxserver/expression.c	2013-09-12 21:07:52.957007976 +0300
@@ -167,4 +167,5 @@
 	double	value1, value2;
 	char	*p, c;
+	int	ret;
 
 	zbx_lrtrim(expression, " ");
@@ -186,4 +187,9 @@
 		return SUCCEED;
 	}
+	else if (0 == strcmp("Undef", expression))
+	{
+		zbx_snprintf(error, maxerrlen, "Undefined function. Exp: [%s]", expression);
+		return FAIL;
+	}
 
 	/* operators with lowest priority come first */
@@ -197,13 +203,43 @@
 		c = *p;
 		*p = '\0';
-
-		if (SUCCEED != evaluate_simple(&value1, expression, error, maxerrlen) ||
-				SUCCEED != evaluate_simple(&value2, p + 1, error, maxerrlen))
-		{
-			*p = c;
-			return FAIL;
+		ret = evaluate_simple(&value1, expression, error, maxerrlen);
+		*p = c;
+		if ('&' == c || '|' == c)
+		{	/* special case: "true | undef" is "true", "false & undef" is "false" */
+			if (SUCCEED == ret)
+			{
+				if ('|' == c && SUCCEED != cmp_double(value1, 0))
+				{
+					*result = -1; /* true */
+					return SUCCEED;
+				}
+				if ('&' == c && SUCCEED == cmp_double(value1, 0))
+				{
+					*result = 0; /* false */
+					return SUCCEED;
+				}
+			}
+			else
+			{
+				if (SUCCEED != evaluate_simple(&value2, p + 1, error, maxerrlen))
+					return FAIL;
+				if ('|' == c && SUCCEED != cmp_double(value2, 0))
+				{
+					*result = -1; /* true */
+					return SUCCEED;
+				}
+				if ('&' == c && SUCCEED == cmp_double(value2, 0))
+				{
+					*result = 0; /* false */
+					return SUCCEED;
+				}
+				return FAIL;
+			}
 		}
+		else if (SUCCEED != ret)
+			return FAIL;
 
-		*p = c;
+		if (SUCCEED != evaluate_simple(&value2, p + 1, error, maxerrlen))
+			return FAIL;
 	}
 	else
@@ -268,5 +304,5 @@
  * Author: Alexei Vladishev                                                   *
  *                                                                            *
- * Comments: example: ({15}>10)|({123}=1)                                     *
+ * Comments: example: (6.456>10)|(0=0)                                        *
  *                                                                            *
  ******************************************************************************/
@@ -3790,8 +3826,5 @@
 
 			if (SUCCEED != evaluate_function(value, &item, func->function, func->parameter, func->timespec.sec))
-			{
-				func->error = zbx_dsprintf(func->error, "Evaluation failed for function: {%s:%s.%s(%s)}",
-						item.host_name, item.key, func->function, func->parameter);
-			}
+				func->value = zbx_strdup(func->value, "Undef");
 			else
 				func->value = zbx_strdup(func->value, value);