diff --git a/changelog.md b/changelog.md
index bc8ad22952..3bdf8a5117 100644
--- a/changelog.md
+++ b/changelog.md
@@ -35,6 +35,8 @@
 
 ## Language changes
 
+- The `cstring` doesn't support `[]=` operator in JS backend.
+
 
 
 ## Compiler changes
diff --git a/compiler/jsgen.nim b/compiler/jsgen.nim
index 87021de111..357e180003 100644
--- a/compiler/jsgen.nim
+++ b/compiler/jsgen.nim
@@ -1049,6 +1049,10 @@ proc genAsgnAux(p: PProc, x, y: PNode, noCopyNeeded: bool) =
   var a, b: TCompRes
   var xtyp = mapType(p, x.typ)
 
+  # disable `[]=` for cstring
+  if x.kind == nkBracketExpr and x.len >= 2 and x[0].typ.skipTypes(abstractInst).kind == tyCString:
+    localError(p.config, x.info, "cstring doesn't support `[]=` operator")
+
   gen(p, x, a)
   genLineDir(p, y)
   gen(p, y, b)
diff --git a/tests/js/tmodify_cstring.nim b/tests/js/tmodify_cstring.nim
new file mode 100644
index 0000000000..82f8ccb232
--- /dev/null
+++ b/tests/js/tmodify_cstring.nim
@@ -0,0 +1,6 @@
+discard """
+  errormsg: "cstring doesn't support `[]=` operator"
+"""
+
+var x = cstring"abcd"
+x[0] = 'x'