diff --git a/packages/cosmic-proto/src/address-hooks.js b/packages/cosmic-proto/src/address-hooks.js
index 819a38a2251..aaa119f43a1 100644
--- a/packages/cosmic-proto/src/address-hooks.js
+++ b/packages/cosmic-proto/src/address-hooks.js
@@ -23,7 +23,7 @@
  *     const decoded = decodeAddressHook(addressHook);
  *     // {
  *     //   baseAddress: 'agoric1qqp0e5ys',
- *     //   query: [Object: null prototype] { foo: [ 'bar', 'baz' ], key: 'value' }
+ *     //   query: { foo: [ 'bar', 'baz' ], key: 'value' }
  *     // }
  */
 
@@ -31,6 +31,10 @@
 import { bech32 } from 'bech32';
 import queryString from 'query-string';
 
+/* global globalThis */
+/** @type {<T>(x: T) => T} */
+const harden = globalThis.harden || Object.freeze;
+
 // ADDRESS_HOOK_VERSION is the version of the address hook format used in this
 // module.
 const ADDRESS_HOOK_VERSION = 0;
@@ -46,6 +50,7 @@ const ADDRESS_HOOK_BYTE_PREFIX = [
   0xf1,
   0x70, // | ADDRESS_HOOK_VERSION
 ];
+harden(ADDRESS_HOOK_BYTE_PREFIX);
 
 /**
  * The default maximum number of characters in a bech32-encoded hooked address.
@@ -63,6 +68,9 @@ export const DEFAULT_HOOKED_ADDRESS_CHAR_LIMIT = 1024;
  *      { key: ['value1', null, 'value3'] } // '?key=value1&key&key=value3'
  */
 
+/**
+ * How many bytes are used to store the length of the base address.
+ */
 export const BASE_ADDRESS_LENGTH_BYTES = 2;
 
 /**
@@ -78,8 +86,9 @@ export const decodeBech32 = (
   const rawBytes = bech32.fromWords(words);
 
   const bytes = new Uint8Array(rawBytes);
-  return { prefix, bytes };
+  return harden({ prefix, bytes });
 };
+harden(decodeBech32);
 
 /**
  * @param {string} humanReadablePart
@@ -95,6 +104,7 @@ export const encodeBech32 = (
   const words = bech32.toWords(bytes);
   return bech32.encode(humanReadablePart, words, charLimit);
 };
+harden(encodeBech32);
 
 /**
  * Join raw base address bytes and hook data into a bech32-encoded hooked
@@ -162,6 +172,7 @@ export const joinHookedAddress = (
 
   return encodeBech32(prefix, hookBuf, charLimit);
 };
+harden(joinHookedAddress);
 
 /**
  * @param {string} baseAddress
@@ -175,6 +186,7 @@ export const encodeAddressHook = (baseAddress, query, charLimit) => {
   const hookData = te.encode(`?${queryStr}`);
   return joinHookedAddress(baseAddress, hookData, charLimit);
 };
+harden(encodeAddressHook);
 
 /**
  * @param {string} addressHook
@@ -188,10 +200,15 @@ export const decodeAddressHook = (addressHook, charLimit) => {
     throw Error(`Hook data does not start with '?': ${hookStr}`);
   }
 
-  /** @type {HookQuery} */
-  const query = queryString.parse(hookStr);
-  return { baseAddress, query };
+  const parsedQuery = queryString.parse(hookStr);
+
+  /**
+   * @type {HookQuery}
+   */
+  const query = harden({ ...parsedQuery });
+  return harden({ baseAddress, query });
 };
+harden(decodeAddressHook);
 
 /**
  * @param {string} specimen
@@ -214,7 +231,7 @@ export const splitHookedAddress = (
       maybeMagicByte &= 0xf0;
     }
     if (maybeMagicByte !== ADDRESS_HOOK_BYTE_PREFIX[i]) {
-      return { baseAddress: specimen, hookData: new Uint8Array() };
+      return harden({ baseAddress: specimen, hookData: new Uint8Array() });
     }
   }
 
@@ -246,5 +263,6 @@ export const splitHookedAddress = (
 
   const hookData = bytes.subarray(prefixLength + b, -BASE_ADDRESS_LENGTH_BYTES);
 
-  return { baseAddress, hookData };
+  return harden({ baseAddress, hookData });
 };
+harden(splitHookedAddress);