These facts lead us to hold that the present case falls within one of the recognized exceptions to the parole evidence rule.  Under the Rules of Court, a party may present evidence to modify, explain or add to the terms of the written agreement if he puts in issue in his pleading, among others, its failure to express the true intent and agreement of the parties thereto.^[15] Here, the contractual intention of the parties cannot be understood from a mere reading of the instrument.  Thus, while the contract explicitly stipulated that it was for the insurance of the new oil mill, the boundary description written on the policy concededly pertains to the first oil mill.  This irreconcilable difference can only be clarified by admitting evidence aliunde, which will explain the imperfection and clarify the intent of the parties.

Anent petitioner's argument that the respondent is barred by estoppel from claiming that the description of the insured oil mill in the policy was wrong, we find that the same proceeds from a wrong assumption.  Evidence on record reveals that respondent's operating manager, Mr. Edison Tantuco, notified Mr. Borja (the petitioner's agent with whom respondent negotiated for the contract) about the inaccurate description in the policy.  However, Mr. Borja assured Mr. Tantuco that the use of the adjective new will distinguish the insured property.  The assurance convinced respondent that, despite the impreciseness in the specification of the boundaries, the insurance will cover the new oil mill