diff --git a/packages/sdk/src/leb128.spec.ts b/packages/sdk/src/leb128.spec.ts
new file mode 100644
index 00000000..c95f8e38
--- /dev/null
+++ b/packages/sdk/src/leb128.spec.ts
@@ -0,0 +1,39 @@
+import { Encoding } from "@iov/encoding";
+
+const { fromHex } = Encoding;
+
+export function leb128Encode(uint: number): Uint8Array {
+  if (uint < 0) throw new Error("Only non-negative values supported");
+  if (uint > 0x7fffffff) throw new Error("Only values in signed int32 range allowed");
+  const out = new Array<number>();
+  let value = uint;
+  do {
+    // tslint:disable: no-bitwise
+    let byte = value & 0b01111111;
+    value >>= 7;
+
+    // more bytes to come: set high order bit of byte
+    if (value !== 0) byte ^= 0b10000000;
+
+    out.push(byte);
+    // tslint:enable: no-bitwise
+  } while (value !== 0);
+  return new Uint8Array(out);
+}
+
+describe("leb128", () => {
+  describe("leb128Encode", () => {
+    it("works for single byte values", () => {
+      // Values in 7 bit range are encoded as one byte
+      expect(leb128Encode(0)).toEqual(fromHex("00"));
+      expect(leb128Encode(20)).toEqual(fromHex("14"));
+      expect(leb128Encode(127)).toEqual(fromHex("7f"));
+    });
+
+    it("works for multi byte values", () => {
+      // from external souce (wasm-objdump)
+      expect(leb128Encode(145)).toEqual(fromHex("9101"));
+      expect(leb128Encode(1539)).toEqual(fromHex("830c"));
+    });
+  });
+});
diff --git a/packages/sdk/src/restclient.spec.ts b/packages/sdk/src/restclient.spec.ts
index d97f9b0e..c65ad70f 100644
--- a/packages/sdk/src/restclient.spec.ts
+++ b/packages/sdk/src/restclient.spec.ts
@@ -4,6 +4,7 @@ import { Encoding } from "@iov/encoding";
 import { HdPaths, Secp256k1HdWallet } from "@iov/keycontrol";
 
 import { encodeSecp256k1Signature, makeSignBytes, marshalTx } from "./encoding";
+import { leb128Encode } from "./leb128.spec";
 import { Log, parseLogs } from "./logs";
 import { RestClient } from "./restclient";
 import contract from "./testdata/contract.json";
@@ -19,7 +20,7 @@ import {
   StdTx,
 } from "./types";
 
-const { fromBase64 } = Encoding;
+const { fromBase64, toBase64 } = Encoding;
 
 const httpUrl = "http://localhost:1317";
 const defaultNetworkId = "testing";
@@ -49,6 +50,32 @@ function makeSignedTx(firstMsg: Msg, fee: StdFee, memo: string, firstSignature:
   };
 }
 
+function getRandomizedContract(): Uint8Array {
+  const data = fromBase64(contract.data);
+  // The return value of the export function cosmwasm_api_0_6 is unused and
+  // can be randomized for testing.
+  //
+  // Find position of mutable bytes as follows:
+  // $ wasm-objdump -d contract.wasm | grep -F "cosmwasm_api_0_6" -A 1
+  // 00e67c func[149] <cosmwasm_api_0_6>:
+  // 00e67d: 41 83 0c                   | i32.const 1539
+  //
+  // In the last line, the addresses 00e67d-00e67f hold a one byte instruction
+  // (https://github.com/WebAssembly/design/blob/master/BinaryEncoding.md#constants-described-here)
+  // and a two byte value (leb128 encoded 1539)
+
+  // Any unsigned integer from 128 to 16383 is encoded to two leb128 bytes
+  const min = 128;
+  const max = 16383;
+  const random = Math.floor(Math.random() * (max - min)) + min;
+  const bytes = leb128Encode(random);
+
+  data[0x00e67d + 1] = bytes[0];
+  data[0x00e67d + 2] = bytes[1];
+
+  return data;
+}
+
 describe("RestClient", () => {
   it("can be constructed", () => {
     const client = new RestClient(httpUrl);
@@ -133,6 +160,8 @@ describe("RestClient", () => {
       const signer = await wallet.createIdentity("abc" as ChainId, faucetPath);
       const client = new RestClient(httpUrl);
 
+      let codeId: number;
+
       // upload
       {
         const memo = "My first contract on chain";
@@ -140,7 +169,7 @@ describe("RestClient", () => {
           type: "wasm/store-code",
           value: {
             sender: faucetAddress,
-            wasm_byte_code: contract.data,
+            wasm_byte_code: toBase64(getRandomizedContract()),
             source: "https://github.com/confio/cosmwasm/raw/0.7/lib/vm/testdata/contract_0.6.wasm",
             builder: "cosmwasm-opt:0.6.2",
           },
@@ -165,7 +194,10 @@ describe("RestClient", () => {
         expect(result.code).toBeFalsy();
         const [firstLog] = parseSuccess(result.raw_log);
         const codeIdAttr = firstLog.events[0].attributes.find(attr => attr.key === "code_id");
-        expect(codeIdAttr).toEqual({ key: "code_id", value: "1" });
+        if (!codeIdAttr) throw new Error("Could not find code_id attribute");
+        codeId = Number.parseInt(codeIdAttr.value, 10);
+        expect(codeId).toBeGreaterThanOrEqual(1);
+        expect(codeId).toBeLessThanOrEqual(200);
       }
 
       let contractAddress: string;
@@ -187,7 +219,7 @@ describe("RestClient", () => {
           type: "wasm/instantiate",
           value: {
             sender: faucetAddress,
-            code_id: "1",
+            code_id: codeId.toString(),
             init_msg: {
               verifier: "cosmos1ltkhnmdcqemmd2tkhnx7qx66tq7e0wykw2j85k",
               beneficiary: "cosmos1ltkhnmdcqemmd2tkhnx7qx66tq7e0wykw2j85k",