diff --git a/Mantle/MTLModel.m b/Mantle/MTLModel.m
index 6cf332cc..2ef60fed 100644
--- a/Mantle/MTLModel.m
+++ b/Mantle/MTLModel.m
@@ -184,8 +184,9 @@ - (void)mergeValueForKey:(NSString *)key fromModel:(MTLModel *)model {
 }
 
 - (void)mergeValuesForKeysFromModel:(MTLModel *)model {
+	NSSet *propertyKeys = model.class.propertyKeys;
 	for (NSString *key in self.class.propertyKeys) {
-		if (![model.class.propertyKeys containsObject:key]) continue;
+		if (![propertyKeys containsObject:key]) continue;
 
 		[self mergeValueForKey:key fromModel:model];
 	}