diff --git a/chemformula.sty b/chemformula.sty
index 3a7b1d9..c16ba3a 100644
--- a/chemformula.sty
+++ b/chemformula.sty
@@ -2324,7 +2324,7 @@
       {
         \tl_if_blank:VF \l__chemformula_stoich_tl
           {
-            \tl_put_left:Nn \l__chemformula_stoich_tl { \q_mark }
+            \tl_put_left:Nn \l__chemformula_stoich_tl { \s_stop }
             \__chemformula_sanitize:NV
               \l__chemformula_stoich_tl
               \l__chemformula_stoich_tl
@@ -2355,7 +2355,7 @@
     \group_end:
   }
 
-\cs_new_protected:Npn \__chemformula_parse_stoich:w \q_mark #1 \q_nil
+\cs_new_protected:Npn \__chemformula_parse_stoich:w \s_stop #1 \q_nil
   {
     \__chemformula_bool_set_if_first_last:Nnnn
       \l__chemformula_stoich_is_iupac_bool {#1} { ( } { ) }