diff --git a/org.adempiere.base/src/org/compiere/model/MOrder.java b/org.adempiere.base/src/org/compiere/model/MOrder.java
index 773133e89e..aaf0d65c7f 100644
--- a/org.adempiere.base/src/org/compiere/model/MOrder.java
+++ b/org.adempiere.base/src/org/compiere/model/MOrder.java
@@ -1093,6 +1093,14 @@ public class MOrder extends X_C_Order implements DocAction
 			}
 		}
 
+		// IDEMPIERE-4318 Validation - Prepay Order must not allow Cash payment rule
+		MDocType dt = MDocType.get(getCtx(), getC_DocTypeTarget_ID());
+		if (   MDocType.DOCSUBTYPESO_PrepayOrder.equals(dt.getDocSubTypeSO())
+			&& PAYMENTRULE_Cash.equals(getPaymentRule())) {
+			log.saveError("Error", Msg.parseTranslation(getCtx(), "@Invalid@ @PaymentRule@"));
+			return false;
+		}
+
 		if (! recursiveCall && (!newRecord && is_ValueChanged(COLUMNNAME_C_PaymentTerm_ID))) {
 			recursiveCall = true;
 			try {