diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 29df4a009c..0a0d00c964 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,7 @@
+2018-07-16  Tom Tromey  <tom@tromey.com>
+
+	* buildsym.c (prepare_for_building): Add assert.
+
 2018-07-16  Tom Tromey  <tom@tromey.com>
 
 	* buildsym.c (~buildsym_compunit): Update.
diff --git a/gdb/buildsym.c b/gdb/buildsym.c
index b548c52e41..035bdb27c0 100644
--- a/gdb/buildsym.c
+++ b/gdb/buildsym.c
@@ -1021,6 +1021,7 @@ prepare_for_building (const char *name, CORE_ADDR start_addr)
   gdb_assert (pending_macros == NULL);
   gdb_assert (pending_addrmap == NULL);
   gdb_assert (current_subfile == NULL);
+  gdb_assert (buildsym_compunit == nullptr);
 }
 
 /* Start a new symtab for a new source file in OBJFILE.  Called, for example,