diff --git a/pic2plot/gram.yy b/pic2plot/gram.yy
index d11320b..1bcce46 100644
--- a/pic2plot/gram.yy
+++ b/pic2plot/gram.yy
@@ -1833,7 +1833,7 @@ do_sprintf(const char *form, const double *v, int nv)
 	    {
 	      one_format += *form++;
 	      one_format += '\0';
-	      sprintf(sprintf_buf, one_format.contents());
+	      sprintf(sprintf_buf, "%s", one_format.contents());
 	    }
 	  else 
 	    {