diff --git a/A00_Common/formula_doc_math_model.ts b/A00_Common/formula_doc_math_model.ts
index ba6aec420..69f88fd26 100644
--- a/A00_Common/formula_doc_math_model.ts
+++ b/A00_Common/formula_doc_math_model.ts
@@ -176,7 +176,7 @@ export function toLinear(items: Item[]): string {
   const go = (list: Item[]): string =>
     list
       .map((it) => {
-        if (it.t === "ch") return OPS.has(it.c) ? ` ${it.c} ` : it.c;
+        if (it.t === "ch") return it.c === "," ? ", " : OPS.has(it.c) ? ` ${it.c} ` : it.c;
         if (it.t === "frac") return `(${go(it.num)}) / (${go(it.den)})`;
         if (it.t === "paren") return `(${go(it.body)})`;
         const body = go(it.body).trim();