Thanks Michael for the immediate response.   Yes.The rule has been defined in guided web editor.   I had taken sample respoitory ( Pricing Loan guided web editor rules) and added the negate option in the row-1 of the rules and Validating the rules throws the error posted earlier.   Here is the source of the rule-1 ,  1. | #from row number: 1 2. | rule "Row 1 Pricing loans" 3. |     dialect "mvel" 4. |     when 5. |         not ( 6. |             application : LoanApplication( amount > "131000" , amount <= "200000" , lengthYears == "30" , deposit < "20000" ) 7. |             income : IncomeSource( type == "Asset" ) 8. |         ) 9. |     then 10. |         application.setApproved( true ); 11. |         application.setInsuranceCost( 0 ); 12. |         application.setApprovedRate( 2 ); 13. | end   Regards, Mahmood From: mahmood lebbai <mahmoodlebbai(a)yahoo.com&gt; To: &quot;rules-users(a)lists.jboss.org&quot; <rules-users(a)lists.jboss.org&gt; Sent: Saturday, 21 May 2011 2:55 PM Subject: DROOLS Negate Question I need help to clarify the following I had taken latest Guvnour 5.2.0 M2 version and use the sample pricing loan rules. When i tried to use the "Negate" Meta data option and set it to true on first rule , i had got the following error. Pricing loans] [ERR 102] Line 7:3 mismatched input 'income' in rule "Row 1 Pricing loans" [Pricing loans] Line 16:0 unexpected exception at input 'rule'. Exception: java.util.EmptyStackException. Stack trace: java.util.EmptyStackException at java.util.Stack.peek(Stack.java:79) at java.util.Stack.pop(Stack.java:61) at org.drools.lang.ParserHelper.popParaphrases(ParserHelper.java:409) at org.drools.lang.ParserHelper.end(ParserHelper.java:723) at org.drools.lang.DRLParser.rule(DRLParser.java:915) at org.drools.lang.DRLParser.statement(DRLParser.java:248) at org.drools.lang.DRLParser.compilationUnit(DRLParser.java:143) at org.drools.compiler.DrlParser.compile(DrlParser.java:225) at org.drools.compiler.DrlParser.parse(DrlParser.java:71) at org.drools.compiler.DrlParser.parse(DrlParser.java:76) at  I have a similar requirement where in I need to write both negate with true and false within the same ruleset in guvnour. Please help me out how to achieve this. Regards, Mahmood