diff --git a/src/transaction_builder.js b/src/transaction_builder.js
index f6a8625..098e6c1 100644
--- a/src/transaction_builder.js
+++ b/src/transaction_builder.js
@@ -166,7 +166,7 @@ class TransactionBuilder {
           `Unknown prevOutScriptType "${signParams.prevOutScriptType}"`,
         );
       }
-      typeforce(typeforce.tuple(typeforce.Number, typeforce.Object), [
+      typeforce(typeforce.tuple(typeforce.Number, types.Signer), [
         signParams.vin,
         signParams.keyPair,
       ]);
diff --git a/src/types.js b/src/types.js
index be95266..8bcee2c 100644
--- a/src/types.js
+++ b/src/types.js
@@ -13,6 +13,14 @@ exports.BIP32Path = BIP32Path;
 BIP32Path.toJSON = () => {
   return 'BIP32 derivation path';
 };
+function Signer(obj) {
+  return (
+    (typeforce.Buffer(obj.publicKey) ||
+      typeof obj.getPublicKey === 'function') &&
+    typeof obj.sign === 'function'
+  );
+}
+exports.Signer = Signer;
 const SATOSHI_MAX = 21 * 1e14;
 function Satoshi(value) {
   return typeforce.UInt53(value) && value <= SATOSHI_MAX;
diff --git a/ts_src/transaction_builder.ts b/ts_src/transaction_builder.ts
index 33ea48b..a2ccc8b 100644
--- a/ts_src/transaction_builder.ts
+++ b/ts_src/transaction_builder.ts
@@ -256,7 +256,7 @@ export class TransactionBuilder {
           `Unknown prevOutScriptType "${signParams.prevOutScriptType}"`,
         );
       }
-      typeforce(typeforce.tuple(typeforce.Number, typeforce.Object), [
+      typeforce(typeforce.tuple(typeforce.Number, types.Signer), [
         signParams.vin,
         signParams.keyPair,
       ]);
diff --git a/ts_src/types.ts b/ts_src/types.ts
index 06c247d..2e41267 100644
--- a/ts_src/types.ts
+++ b/ts_src/types.ts
@@ -12,6 +12,14 @@ BIP32Path.toJSON = (): string => {
   return 'BIP32 derivation path';
 };
 
+export function Signer(obj: any): boolean {
+  return (
+    (typeforce.Buffer(obj.publicKey) ||
+      typeof obj.getPublicKey === 'function') &&
+    typeof obj.sign === 'function'
+  );
+}
+
 const SATOSHI_MAX: number = 21 * 1e14;
 export function Satoshi(value: number): boolean {
   return typeforce.UInt53(value) && value <= SATOSHI_MAX;
diff --git a/types/types.d.ts b/types/types.d.ts
index 242bab8..e7c588d 100644
--- a/types/types.d.ts
+++ b/types/types.d.ts
@@ -3,6 +3,7 @@ export declare function BIP32Path(value: string): boolean;
 export declare namespace BIP32Path {
     var toJSON: () => string;
 }
+export declare function Signer(obj: any): boolean;
 export declare function Satoshi(value: number): boolean;
 export declare const ECPoint: any;
 export declare const Network: any;