diff --git a/package-lock.json b/package-lock.json
index e9059fc..850e840 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -7745,22 +7745,6 @@
         "url": "https://github.com/chalk/strip-ansi?sponsor=1"
       }
     },
-    "node_modules/semantic-release/node_modules/typescript": {
-      "version": "5.9.3",
-      "resolved": "https://registry.npmjs.org/typescript/-/typescript-5.9.3.tgz",
-      "integrity": "sha512-jl1vZzPDinLr9eUt3J/t7V6FgNEw9QjvBPdysz9KfQDD41fQrC2Y4vKQdiaUpFT4bXlb1RHhLpp8wtm6M5TgSw==",
-      "dev": true,
-      "license": "Apache-2.0",
-      "optional": true,
-      "peer": true,
-      "bin": {
-        "tsc": "bin/tsc",
-        "tsserver": "bin/tsserver"
-      },
-      "engines": {
-        "node": ">=14.17"
-      }
-    },
     "node_modules/semantic-release/node_modules/wrap-ansi": {
       "version": "9.0.2",
       "resolved": "https://registry.npmjs.org/wrap-ansi/-/wrap-ansi-9.0.2.tgz",
diff --git a/src/lib/psbt.js b/src/lib/psbt.js
index 5f61863..be68d89 100644
--- a/src/lib/psbt.js
+++ b/src/lib/psbt.js
@@ -112,7 +112,7 @@ class Psbt {
       throw new Error('unknownKeyVals must be an Array');
     }
     addKeyVals.forEach(keyVal =>
-      this.addUnknownKeyValToInput(outputIndex, keyVal),
+      this.addUnknownKeyValToOutput(outputIndex, keyVal),
     );
     utils_1.addOutputAttributes(this.outputs, outputData);
     return this;
diff --git a/src/tests/addInputOutput.js b/src/tests/addInputOutput.js
index f1e2f15..9e94b97 100644
--- a/src/tests/addInputOutput.js
+++ b/src/tests/addInputOutput.js
@@ -3,6 +3,85 @@ Object.defineProperty(exports, '__esModule', { value: true });
 const tape = require('tape');
 const psbt_1 = require('../lib/psbt');
 const txTools_1 = require('./utils/txTools');
+const SCRIPT = Buffer.from(
+  'a914e18870f2c297fbfca54c5c6f645c7745a5b66eda87',
+  'hex',
+);
+// Proprietary key: 0xfc prefix + identifier + subtype (type byte 0xfc is
+// above both InputTypes (16 entries) and OutputTypes (6 entries), so it is
+// always an unknown key for either map).
+const PROPRIETARY_KEY = Buffer.from('fc0a626974676f2f6d7573696700', 'hex');
+const PROPRIETARY_VAL = Buffer.from('deadbeef', 'hex');
+tape(
+  'Test: addOutput routes unknownKeyVals to output map, not input map',
+  t => {
+    const psbt = new psbt_1.Psbt(txTools_1.getDefaultTx());
+    psbt.addInput({
+      hash: '865dce988413971fd812d0e81a3395ed916a87ea533e1a16c0f4e15df96fa7d4',
+      index: 0,
+    });
+    psbt.addOutput({
+      script: SCRIPT,
+      value: 1000000,
+      unknownKeyVals: [{ key: PROPRIETARY_KEY, value: PROPRIETARY_VAL }],
+    });
+    // Key must be on the output, not the input
+    t.deepEqual(
+      psbt.outputs[0].unknownKeyVals,
+      [{ key: PROPRIETARY_KEY, value: PROPRIETARY_VAL }],
+      'output[0].unknownKeyVals contains the proprietary key',
+    );
+    t.deepEqual(
+      psbt.inputs[0].unknownKeyVals,
+      [],
+      'input[0].unknownKeyVals is empty',
+    );
+    // Placement must survive a serialize/parse round-trip
+    const psbt2 = psbt_1.Psbt.fromHex(
+      psbt.toHex(),
+      txTools_1.transactionFromBuffer,
+    );
+    t.deepEqual(
+      psbt2.outputs[0].unknownKeyVals,
+      [{ key: PROPRIETARY_KEY, value: PROPRIETARY_VAL }],
+      'output[0].unknownKeyVals survives round-trip',
+    );
+    // fromBuffer leaves unknownKeyVals undefined when there are none;
+    // either undefined or [] is acceptable here.
+    t.ok(
+      !psbt2.inputs[0].unknownKeyVals ||
+        psbt2.inputs[0].unknownKeyVals.length === 0,
+      'input[0].unknownKeyVals is empty after round-trip',
+    );
+    t.end();
+  },
+);
+tape(
+  'Test: addOutput with unknownKeyVals does not throw when outputs > inputs',
+  t => {
+    // Common case: 1 input, 2 outputs — the second addOutput formerly threw
+    // "No input #1" because it called addUnknownKeyValToInput(1, ...).
+    const psbt = new psbt_1.Psbt(txTools_1.getDefaultTx());
+    psbt.addInput({
+      hash: '865dce988413971fd812d0e81a3395ed916a87ea533e1a16c0f4e15df96fa7d4',
+      index: 0,
+    });
+    psbt.addOutput({ script: SCRIPT, value: 1000000 });
+    t.doesNotThrow(() => {
+      psbt.addOutput({
+        script: SCRIPT,
+        value: 500000,
+        unknownKeyVals: [{ key: PROPRIETARY_KEY, value: PROPRIETARY_VAL }],
+      });
+    }, 'second addOutput with unknownKeyVals must not throw');
+    t.deepEqual(
+      psbt.outputs[1].unknownKeyVals,
+      [{ key: PROPRIETARY_KEY, value: PROPRIETARY_VAL }],
+      'output[1] holds the proprietary key',
+    );
+    t.end();
+  },
+);
 tape('Test: add Input Output', t => {
   const psbt = new psbt_1.Psbt(txTools_1.getDefaultTx());
   psbt.addInput({
diff --git a/ts_src/lib/psbt.ts b/ts_src/lib/psbt.ts
index 68905a3..30565d8 100644
--- a/ts_src/lib/psbt.ts
+++ b/ts_src/lib/psbt.ts
@@ -156,7 +156,7 @@ export class Psbt {
       throw new Error('unknownKeyVals must be an Array');
     }
     addKeyVals.forEach((keyVal: KeyValue) =>
-      this.addUnknownKeyValToInput(outputIndex, keyVal),
+      this.addUnknownKeyValToOutput(outputIndex, keyVal),
     );
     addOutputAttributes(this.outputs, outputData);
     return this;
diff --git a/ts_src/tests/addInputOutput.ts b/ts_src/tests/addInputOutput.ts
index f49d5ab..30f204e 100644
--- a/ts_src/tests/addInputOutput.ts
+++ b/ts_src/tests/addInputOutput.ts
@@ -2,6 +2,92 @@ import * as tape from 'tape';
 import { Psbt } from '../lib/psbt';
 import { getDefaultTx, transactionFromBuffer } from './utils/txTools';
 
+const SCRIPT = Buffer.from(
+  'a914e18870f2c297fbfca54c5c6f645c7745a5b66eda87',
+  'hex',
+);
+
+// Proprietary key: 0xfc prefix + identifier + subtype (type byte 0xfc is
+// above both InputTypes (16 entries) and OutputTypes (6 entries), so it is
+// always an unknown key for either map).
+const PROPRIETARY_KEY = Buffer.from('fc0a626974676f2f6d7573696700', 'hex');
+const PROPRIETARY_VAL = Buffer.from('deadbeef', 'hex');
+
+tape(
+  'Test: addOutput routes unknownKeyVals to output map, not input map',
+  t => {
+    const psbt = new Psbt(getDefaultTx());
+    psbt.addInput({
+      hash: '865dce988413971fd812d0e81a3395ed916a87ea533e1a16c0f4e15df96fa7d4',
+      index: 0,
+    });
+    psbt.addOutput({
+      script: SCRIPT,
+      value: 1000000,
+      unknownKeyVals: [{ key: PROPRIETARY_KEY, value: PROPRIETARY_VAL }],
+    });
+
+    // Key must be on the output, not the input
+    t.deepEqual(
+      psbt.outputs[0].unknownKeyVals,
+      [{ key: PROPRIETARY_KEY, value: PROPRIETARY_VAL }],
+      'output[0].unknownKeyVals contains the proprietary key',
+    );
+    t.deepEqual(
+      psbt.inputs[0].unknownKeyVals,
+      [],
+      'input[0].unknownKeyVals is empty',
+    );
+
+    // Placement must survive a serialize/parse round-trip
+    const psbt2 = Psbt.fromHex(psbt.toHex(), transactionFromBuffer);
+    t.deepEqual(
+      psbt2.outputs[0].unknownKeyVals,
+      [{ key: PROPRIETARY_KEY, value: PROPRIETARY_VAL }],
+      'output[0].unknownKeyVals survives round-trip',
+    );
+    // fromBuffer leaves unknownKeyVals undefined when there are none;
+    // either undefined or [] is acceptable here.
+    t.ok(
+      !psbt2.inputs[0].unknownKeyVals ||
+        psbt2.inputs[0].unknownKeyVals.length === 0,
+      'input[0].unknownKeyVals is empty after round-trip',
+    );
+
+    t.end();
+  },
+);
+
+tape(
+  'Test: addOutput with unknownKeyVals does not throw when outputs > inputs',
+  t => {
+    // Common case: 1 input, 2 outputs — the second addOutput formerly threw
+    // "No input #1" because it called addUnknownKeyValToInput(1, ...).
+    const psbt = new Psbt(getDefaultTx());
+    psbt.addInput({
+      hash: '865dce988413971fd812d0e81a3395ed916a87ea533e1a16c0f4e15df96fa7d4',
+      index: 0,
+    });
+    psbt.addOutput({ script: SCRIPT, value: 1000000 });
+
+    t.doesNotThrow(() => {
+      psbt.addOutput({
+        script: SCRIPT,
+        value: 500000,
+        unknownKeyVals: [{ key: PROPRIETARY_KEY, value: PROPRIETARY_VAL }],
+      });
+    }, 'second addOutput with unknownKeyVals must not throw');
+
+    t.deepEqual(
+      psbt.outputs[1].unknownKeyVals,
+      [{ key: PROPRIETARY_KEY, value: PROPRIETARY_VAL }],
+      'output[1] holds the proprietary key',
+    );
+
+    t.end();
+  },
+);
+
 tape('Test: add Input Output', t => {
   const psbt = new Psbt(getDefaultTx());
   psbt.addInput({