diff --git a/packages/compiler/src/frontend/lowering/expressions/binary.ts b/packages/compiler/src/frontend/lowering/expressions/binary.ts
index 9bb74e72b..fafbbf695 100644
--- a/packages/compiler/src/frontend/lowering/expressions/binary.ts
+++ b/packages/compiler/src/frontend/lowering/expressions/binary.ts
@@ -1154,7 +1154,12 @@ export function lowerBinary(lowerer: Lowerer, expr: ts.BinaryExpression): IrExpr
         }
         if (recv?.type.kind === "array" || recv?.type.kind === "bytes") {
           const receiver = lowerer.declareHiddenLocal("%setArray", recv.type);
-          const index = lowerer.lowerExpr(expr.left.argumentExpression);
+          const index = lowerOptionalNumber(
+            lowerer,
+            lowerer.lowerExpr(expr.left.argumentExpression),
+            locOf(expr.left.argumentExpression),
+            expr.left.argumentExpression,
+          );
           if (index.type.kind !== "f64")
             lowerer.unsupported(
               "SC1090",
diff --git a/packages/compiler/src/frontend/lowering/expressions/element-access.ts b/packages/compiler/src/frontend/lowering/expressions/element-access.ts
index 0c4e89f87..74f0be955 100644
--- a/packages/compiler/src/frontend/lowering/expressions/element-access.ts
+++ b/packages/compiler/src/frontend/lowering/expressions/element-access.ts
@@ -1493,7 +1493,12 @@ export function lowerElementCompoundParts(
       "compound assignment through a non-native array or byte view",
     );
   }
-  const index = lowerer.lowerExpr(target.argumentExpression);
+  const index = lowerOptionalNumber(
+    lowerer,
+    lowerer.lowerExpr(target.argumentExpression),
+    locOf(target.argumentExpression),
+    target.argumentExpression,
+  );
   if (index.type.kind !== "f64")
     lowerer.unsupported("SC1090", target.argumentExpression, "indexing with non-number keys");
   const receiverLocal = lowerer.declareHiddenLocal("%compoundArray", receiver.type);
@@ -1836,7 +1841,14 @@ export function lowerElementWrite(lowerer: Lowerer, expr: ts.BinaryExpression):
     const recv = lowerer.lowerExpr(target.expression);
     // The checker's flow type is a typed array, but the storage is wider.
     if (recv.type.kind !== "bytes") return lowerWiderBytesWrite(lowerer, expr, target, recv);
-    const index = lowerer.lowerExpr(target.argumentExpression);
+    // An index the checker types as a number but whose storage may hold
+    // undefined (a hole read) converts with ToNumber, as the reads do.
+    const index = lowerOptionalNumber(
+      lowerer,
+      lowerer.lowerExpr(target.argumentExpression),
+      locOf(target.argumentExpression),
+      target.argumentExpression,
+    );
     if (index.type.kind === "dyn" && recv.type.kind === "bytes") {
       return {
         kind: "exprStmt",
diff --git a/packages/compiler/src/frontend/lowering/expressions/updates.ts b/packages/compiler/src/frontend/lowering/expressions/updates.ts
index 104057906..7269f6b52 100644
--- a/packages/compiler/src/frontend/lowering/expressions/updates.ts
+++ b/packages/compiler/src/frontend/lowering/expressions/updates.ts
@@ -261,7 +261,12 @@ export function lowerIncDec(
       (raw.type.kind === "array" || raw.type.kind === "bytes")
     ) {
       const receiver = save(raw, "%incrementArray");
-      const rawIndex = lowerer.lowerExpr(access.argumentExpression);
+      const rawIndex = lowerOptionalNumber(
+        lowerer,
+        lowerer.lowerExpr(access.argumentExpression),
+        locOf(access.argumentExpression),
+        access.argumentExpression,
+      );
       const index = save(
         rawIndex.type.kind === "dyn" && isJsSourceFile(ts.getSourceFileOfNode(expr))
           ? { kind: "libCall", fn: "dyn.toNumberCoerce", args: [rawIndex], type: F64, loc }
diff --git a/packages/compiler/src/frontend/lowering/lower-stmts.ts b/packages/compiler/src/frontend/lowering/lower-stmts.ts
index df055aed4..14dadfc4d 100644
--- a/packages/compiler/src/frontend/lowering/lower-stmts.ts
+++ b/packages/compiler/src/frontend/lowering/lower-stmts.ts
@@ -194,6 +194,7 @@ import {
   lowerElementCompoundStatement,
   lowerIncDec,
   lowerIncDecTarget,
+  lowerOptionalNumber,
   lowerGroupsProjection,
   matchResultNamedGroupsOf,
   runtimeOptionalGuardIds,
@@ -8835,7 +8836,12 @@ function lowerAssignTargetInto(
       if (arr.type.kind === "array") {
         const recv = lowerer.declareHiddenLocal("%dtRecv", arr.type);
         out.push({ kind: "varDecl", localId: recv.id, init: arr, loc });
-        const index = lowerer.lowerExpr(t.argumentExpression);
+        const index = lowerOptionalNumber(
+          lowerer,
+          lowerer.lowerExpr(t.argumentExpression),
+          locOf(t.argumentExpression),
+          t.argumentExpression,
+        );
         if (index.type.kind !== "f64") {
           lowerer.unsupported("SC1090", t.argumentExpression, "indexing with non-number keys");
         }
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/optional-index-element-writes.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/optional-index-element-writes.ts.json
new file mode 100644
index 000000000..e748f8e75
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/optional-index-element-writes.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/optional-index-element-writes.ts"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/optional-index-element-writes.ts b/tests/corpus/optional-index-element-writes.ts
new file mode 100644
index 000000000..6f55d07a5
--- /dev/null
+++ b/tests/corpus/optional-index-element-writes.ts
@@ -0,0 +1,35 @@
+// Element writes whose index comes from for...of over a number[] field the
+// class also writes by index (so the loop variable's storage may be
+// undefined, a hole read): typed-array writes, compound and update writes,
+// a write used as a value, and a destructuring target all convert the
+// index with ToNumber, as the element reads do.
+class Heap {
+  private k: number[] = [];
+  push(v: number): void {
+    const i = this.k.length;
+    this.k.push(v);
+    this.k[i] = v;
+  }
+  all(): number[] {
+    return this.k;
+  }
+}
+const h = new Heap();
+h.push(2);
+h.push(0);
+h.push(1);
+const cost = new Float64Array(4);
+for (const i of h.all()) cost[i] = i * 1.5;
+console.log(Array.from(cost).join(","));
+const counts = new Int32Array(3);
+for (const i of h.all()) counts[i] += 10;
+for (const i of h.all()) counts[i]++;
+console.log(Array.from(counts).join(","));
+const marks = new Uint8Array(3);
+let last = 0;
+for (const i of h.all()) last = marks[i] = i + 1;
+console.log(Array.from(marks).join(","), last);
+const plain = [0, 0, 0];
+for (const i of h.all()) [plain[i]] = [i * 2];
+for (const i of h.all()) plain[i] += 1;
+console.log(plain.join(","));