diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4554257a..061f1cf8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -20,8 +20,12 @@ and this project adheres to
 
 - @cosmjs/stargate: Handle key value pairs in tx search correctly if the value
   is a numeric value. ([#1462])
+- @cosmjs/cosmwasm-stargate: Make `fix_msg` optional in
+  `AminoMsgInstantiateContract2` and omit default in the Amino JSON converter to
+  fix Amino JSON signing for MsgInstantiateContract2. ([#1509])
 
 [#1462]: https://github.com/cosmos/cosmjs/issues/1462
+[#1509]: https://github.com/cosmos/cosmjs/pull/1509
 
 ### Changed
 
diff --git a/packages/cosmwasm-stargate/src/modules/wasm/aminomessages.spec.ts b/packages/cosmwasm-stargate/src/modules/wasm/aminomessages.spec.ts
index a9413a32..66613a53 100644
--- a/packages/cosmwasm-stargate/src/modules/wasm/aminomessages.spec.ts
+++ b/packages/cosmwasm-stargate/src/modules/wasm/aminomessages.spec.ts
@@ -159,7 +159,7 @@ describe("AminoTypes", () => {
             funds: coins(1234, "ucosm"),
             admin: "cosmos10dyr9899g6t0pelew4nvf4j5c3jcgv0r73qga5",
             salt: toBase64(toUtf8("salt")),
-            fix_msg: false,
+            fix_msg: undefined,
           },
         };
         expect(aminoMsg).toEqual(expected);
@@ -191,7 +191,39 @@ describe("AminoTypes", () => {
             funds: coins(1234, "ucosm"),
             admin: undefined,
             salt: toBase64(toUtf8("salt")),
-            fix_msg: false,
+            fix_msg: undefined,
+          },
+        };
+        expect(aminoMsg).toEqual(expected);
+      }
+
+      // With fixMsg=true (typically not needed)
+      {
+        const msg: MsgInstantiateContract2 = {
+          sender: "cosmos1pkptre7fdkl6gfrzlesjjvhxhlc3r4gmmk8rs6",
+          codeId: BigInt("12345"),
+          label: "sticky",
+          msg: toUtf8(`{"foo":"bar"}`),
+          funds: coins(1234, "ucosm"),
+          admin: "cosmos10dyr9899g6t0pelew4nvf4j5c3jcgv0r73qga5",
+          salt: toUtf8("salt"),
+          fixMsg: true,
+        };
+        const aminoMsg = new AminoTypes(createWasmAminoConverters()).toAmino({
+          typeUrl: "/cosmwasm.wasm.v1.MsgInstantiateContract2",
+          value: msg,
+        });
+        const expected: AminoMsgInstantiateContract2 = {
+          type: "wasm/MsgInstantiateContract2",
+          value: {
+            sender: "cosmos1pkptre7fdkl6gfrzlesjjvhxhlc3r4gmmk8rs6",
+            code_id: "12345",
+            label: "sticky",
+            msg: { foo: "bar" },
+            funds: coins(1234, "ucosm"),
+            admin: "cosmos10dyr9899g6t0pelew4nvf4j5c3jcgv0r73qga5",
+            salt: toBase64(toUtf8("salt")),
+            fix_msg: true,
           },
         };
         expect(aminoMsg).toEqual(expected);
diff --git a/packages/cosmwasm-stargate/src/modules/wasm/aminomessages.ts b/packages/cosmwasm-stargate/src/modules/wasm/aminomessages.ts
index c4e1adb8..ec649494 100644
--- a/packages/cosmwasm-stargate/src/modules/wasm/aminomessages.ts
+++ b/packages/cosmwasm-stargate/src/modules/wasm/aminomessages.ts
@@ -1,4 +1,5 @@
 /* eslint-disable @typescript-eslint/naming-convention */
+import { omitDefault } from "@cosmjs/amino";
 import { fromBase64, fromUtf8, toBase64, toUtf8 } from "@cosmjs/encoding";
 import { AminoConverters, Coin } from "@cosmjs/stargate";
 import {
@@ -141,8 +142,11 @@ export interface AminoMsgInstantiateContract2 {
     readonly admin?: string;
     /** Arbitrary Base64-encoded value provided by the sender */
     readonly salt: string;
-    /** Whether or not to include the msg value into the hash for the address */
-    readonly fix_msg: boolean;
+    /**
+     * Whether or not to include the msg value into the hash for the address.
+     * Unset means false. This should always be unset/false (https://medium.com/cosmwasm/dev-note-3-limitations-of-instantiate2-and-how-to-deal-with-them-a3f946874230).
+     */
+    readonly fix_msg?: boolean;
   };
 }
 
@@ -248,7 +252,7 @@ export function createWasmAminoConverters(): AminoConverters {
         label: label,
         msg: JSON.parse(fromUtf8(msg)),
         funds: funds,
-        admin: admin || undefined,
+        admin: omitDefault(admin),
       }),
       fromAmino: ({
         sender,
@@ -283,9 +287,9 @@ export function createWasmAminoConverters(): AminoConverters {
         label: label,
         msg: JSON.parse(fromUtf8(msg)),
         funds: funds,
-        admin: admin || undefined,
+        admin: omitDefault(admin),
         salt: toBase64(salt),
-        fix_msg: fixMsg,
+        fix_msg: omitDefault(fixMsg),
       }),
       fromAmino: ({
         sender,
@@ -304,7 +308,7 @@ export function createWasmAminoConverters(): AminoConverters {
         funds: [...funds],
         admin: admin ?? "",
         salt: fromBase64(salt),
-        fixMsg: fix_msg,
+        fixMsg: fix_msg ?? false,
       }),
     },
     "/cosmwasm.wasm.v1.MsgUpdateAdmin": {