diff --git a/lib/rb/ext/struct.c b/lib/rb/ext/struct.c
index f500d03..e3aa855 100644
--- a/lib/rb/ext/struct.c
+++ b/lib/rb/ext/struct.c
@@ -290,7 +290,7 @@
 
     if (TYPE(value) == T_ARRAY) {
       items = value;
-    } else {        
+    } else {
       if (rb_cSet == CLASS_OF(value)) {
         items = rb_funcall(value, entries_method_id, 0);
       } else {
@@ -670,6 +670,10 @@
 
   VALUE field_info = rb_hash_aref(struct_fields, field_id);
 
+  if(NIL_P(field_info)) {
+    rb_raise(rb_eRuntimeError, "set_field is not valid for this union!");
+  }
+
   VALUE ttype_value = rb_hash_aref(field_info, type_sym);
   int ttype = FIX2INT(ttype_value);