Merge pull request #814 from gosuri/master

core: fix for #813 - added a gaurd for interface conversion

helper/schema/field_writer_map.go

@@ -293,6 +293,12 @@ func (w *MapFieldWriter) setSet(
 	}
 
 	k := strings.Join(addr, ".")
+
+	if value == nil {
+		w.result[k+".#"] = "0"
+		return nil
+	}
+
 	for code, elem := range value.(*Set).m {
 		codeStr := strconv.FormatInt(int64(code), 10)
 		if err := w.set(append(addrCopy, codeStr), elem); err != nil {
@@ -302,5 +308,4 @@ func (w *MapFieldWriter) setSet(
 
 	w.result[k+".#"] = strconv.Itoa(value.(*Set).Len())
 	return nil
-
 }