diff --git a/src/hotp.js b/src/hotp.js
index 86be771..3931da3 100644
--- a/src/hotp.js
+++ b/src/hotp.js
@@ -50,6 +50,10 @@ export class HOTP extends OTP {
   verify(otp, counter) {
     const otp_count = this.at(counter);
 
+    if (typeof(otp) === 'number') {
+      otp = otp.toString();
+    }
+
     if (otp === otp_count) {
       return true;
     }
diff --git a/src/totp.js b/src/totp.js
index d926314..d76ea7d 100644
--- a/src/totp.js
+++ b/src/totp.js
@@ -75,6 +75,10 @@ export class TOTP extends OTP {
     }
     otp_time = super.generate_otp(Util.timecode(time, this.interval));
 
+    if (typeof(otp) === 'number') {
+      otp = otp.toString();
+    }
+
     if (otp === otp_time) {
       return true;
     }