diff --git a/bootstrap.conf b/bootstrap.conf
index 2450ff07..1e58284a 100644
--- a/bootstrap.conf
+++ b/bootstrap.conf
@@ -17,7 +17,7 @@
 
 # gnulib modules used by this package.
 gnulib_modules='
-  argmatch array-list assert
+  argmatch array-list assert assure
   bitsetv
   calloc-posix close closeout config-h c-strcase
   configmake
diff --git a/lib/.gitignore b/lib/.gitignore
index 919f23c4..b6fc6fe8 100644
--- a/lib/.gitignore
+++ b/lib/.gitignore
@@ -318,3 +318,4 @@
 /rename.c
 /rmdir.c
 /same-inode.h
+/assure.h
diff --git a/src/symtab.c b/src/symtab.c
index 0941d2b5..14c672b6 100644
--- a/src/symtab.c
+++ b/src/symtab.c
@@ -23,6 +23,7 @@
 
 #include "system.h"
 
+#include <assure.h>
 #include <hash.h>
 
 #include "complain.h"
@@ -865,9 +866,9 @@ dummy_symbol_get (location loc)
 {
   /* Incremented for each generated symbol.  */
   static int dummy_count = 0;
-  static char buf[256];
-
-  sprintf (buf, "$@%d", ++dummy_count);
+  char buf[32];
+  int len = snprintf (buf, sizeof buf, "$@%d", ++dummy_count);
+  assure (len < sizeof buf);
   symbol *sym = symbol_get (buf, loc);
   sym->content->class = nterm_sym;
   sym->content->number = nvars++;