diff --git a/ext/objspace/objspace_dump.c b/ext/objspace/objspace_dump.c
index 602cbadef5..7dcee589bf 100644
--- a/ext/objspace/objspace_dump.c
+++ b/ext/objspace/objspace_dump.c
@@ -27,12 +27,12 @@ struct dump_config {
     VALUE type;
     FILE *stream;
     VALUE string;
-    int roots;
     const char *root_category;
     VALUE cur_obj;
     VALUE cur_obj_klass;
     size_t cur_obj_references;
-    int full_heap;
+    unsigned int roots: 1;
+    unsigned int full_heap: 1;
 };
 
 PRINTF_ARGS(static void dump_append(struct dump_config *, const char *, ...), 2, 3);
@@ -368,7 +368,7 @@ root_obj_i(const char *category, VALUE obj, void *data)
         dump_append(dc, ", \"%#"PRIxVALUE"\"", obj);
 
     dc->root_category = category;
-    dc->roots++;
+    dc->roots = 1;
 }
 
 static VALUE