diff --git a/momcom/NSPropertyDescription+momcom.m b/momcom/NSPropertyDescription+momcom.m
index 1e9841c..d851b49 100644
--- a/momcom/NSPropertyDescription+momcom.m
+++ b/momcom/NSPropertyDescription+momcom.m
@@ -19,6 +19,9 @@ + (id)baseEntityForXML:(NSXMLElement *)xmlNode
     }
 
     NSPropertyDescription *propertyDescription = [[self alloc] init];
+    // set optional to `NO`, unless it appears in XML attributes
+    propertyDescription.optional = NO;
+
     BOOL syncable = NO;
 
     for (NSXMLNode *xmlAttribute in [xmlNode attributes]) {