diff --git a/jwt/api_jws.py b/jwt/api_jws.py
index 0c61c7df..3e79d41a 100644
--- a/jwt/api_jws.py
+++ b/jwt/api_jws.py
@@ -5,8 +5,8 @@
 from collections import Mapping
 
 from .algorithms import Algorithm, get_default_algorithms  # NOQA
-from .compat import text_type
-from .exceptions import DecodeError, InvalidAlgorithmError
+from .compat import string_types, text_type
+from .exceptions import DecodeError, InvalidAlgorithmError, InvalidTokenError
 from .utils import base64url_decode, base64url_encode, merge_dict
 
 
@@ -79,6 +79,7 @@ def encode(self, payload, key, algorithm='HS256', headers=None,
         header = {'typ': self.header_typ, 'alg': algorithm}
 
         if headers:
+            self._validate_headers(headers)
             header.update(headers)
 
         json_header = json.dumps(
@@ -125,7 +126,10 @@ def get_unverified_header(self, jwt):
         Note: The signature is not verified so the header parameters
         should not be fully trusted until signature verification is complete
         """
-        return self._load(jwt)[2]
+        headers = self._load(jwt)[2]
+        self._validate_headers(headers)
+
+        return headers
 
     def _load(self, jwt):
         if isinstance(jwt, text_type):
@@ -180,6 +184,13 @@ def _verify_signature(self, payload, signing_input, header, signature,
         except KeyError:
             raise InvalidAlgorithmError('Algorithm not supported')
 
+    def _validate_headers(self, headers):
+        if 'kid' in headers:
+            self._validate_kid(headers['kid'])
+
+    def _validate_kid(self, kid):
+        if not isinstance(kid, string_types):
+            raise InvalidTokenError('Key ID header parameter must be a string')
 
 _jws_global_obj = PyJWS()
 encode = _jws_global_obj.encode
diff --git a/tests/test_api_jws.py b/tests/test_api_jws.py
index 9aa8b85b..2079bef0 100644
--- a/tests/test_api_jws.py
+++ b/tests/test_api_jws.py
@@ -6,7 +6,7 @@
 from jwt.algorithms import Algorithm
 from jwt.api_jws import PyJWS
 from jwt.exceptions import (
-    DecodeError, InvalidAlgorithmError
+    DecodeError, InvalidAlgorithmError, InvalidTokenError
 )
 from jwt.utils import base64url_decode
 
@@ -367,12 +367,24 @@ def test_decode_with_algo_none_and_verify_false_should_pass(self, jws, payload):
 
     def test_get_unverified_header_returns_header_values(self, jws, payload):
         jws_message = jws.encode(payload, key='secret', algorithm='HS256',
-                                 headers={'kid': 123})
+                                 headers={'kid': 'toomanysecrets'})
 
         header = jws.get_unverified_header(jws_message)
 
         assert 'kid' in header
-        assert header['kid'] == 123
+        assert header['kid'] == 'toomanysecrets'
+
+    def test_get_unverified_header_fails_on_bad_header_types(self, jws, payload):
+        # Contains a bad kid value (int 123 instead of string)
+        example_jws = (
+            'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCIsImtpZCI6MTIzfQ'
+            '.eyJzdWIiOiIxMjM0NTY3ODkwIn0'
+            '.vs2WY54jfpKP3JGC73Vq5YlMsqM5oTZ1ZydT77SiZSk')
+
+        with pytest.raises(InvalidTokenError) as exc:
+            jws.get_unverified_header(example_jws)
+
+        assert 'Key ID header parameter must be a string' == str(exc.value)
 
     @pytest.mark.skipif(not has_crypto, reason='Not supported without cryptography library')
     def test_encode_decode_with_rsa_sha256(self, jws, payload):
@@ -597,3 +609,14 @@ def test_encode_headers_parameter_adds_headers(self, jws, payload):
 
         assert 'testheader' in header_obj
         assert header_obj['testheader'] == headers['testheader']
+
+    def test_encode_fails_on_invalid_kid_types(self, jws, payload):
+        with pytest.raises(InvalidTokenError) as exc:
+            jws.encode(payload, 'secret', headers={'kid': 123})
+
+        assert 'Key ID header parameter must be a string' == str(exc.value)
+
+        with pytest.raises(InvalidTokenError) as exc:
+            jws.encode(payload, 'secret', headers={'kid': None})
+
+        assert 'Key ID header parameter must be a string' == str(exc.value)