diff --git a/requests/utils.py b/requests/utils.py
index dea323ef..397a655e 100644
--- a/requests/utils.py
+++ b/requests/utils.py
@@ -739,8 +739,9 @@ _CLEAN_HEADER_REGEX_BYTE = re.compile(b'^\\S[^\\r\\n]*$|^$')
 _CLEAN_HEADER_REGEX_STR = re.compile(r'^\S[^\r\n]*$|^$')
 
 def check_header_validity(header):
-    """Verifies that header value doesn't contain leading whitespace or
-    return characters. This prevents unintended header injection.
+    """Verifies that header value is a string which doesn't contain 
+    leading whitespace or return characters. This prevents unintended
+    header injection.
 
     :param header: tuple, in the format (name, value).
     """
@@ -750,8 +751,12 @@ def check_header_validity(header):
         pat = _CLEAN_HEADER_REGEX_BYTE
     else:
         pat = _CLEAN_HEADER_REGEX_STR
-    if not pat.match(value):
-        raise InvalidHeader("Invalid return character or leading space in header: %s" % name)
+    try:
+        if not pat.match(value):
+            raise InvalidHeader("Invalid return character or leading space in header: %s" % name)
+    except TypeError:
+        raise InvalidHeader("Header value %s must be of type str or bytes, " 
+                            "not %s" % (value, type(value)))
 
 def urldefragauth(url):
     """
diff --git a/tests/test_requests.py b/tests/test_requests.py
index 4250a8f9..a7d3a75b 100755
--- a/tests/test_requests.py
+++ b/tests/test_requests.py
@@ -1142,15 +1142,33 @@ class TestRequests:
         assert 'unicode' in p.headers.keys()
         assert 'byte' in p.headers.keys()
 
-    def test_header_validation(self,httpbin):
+    def test_header_validation(self, httpbin):
         """Ensure prepare_headers regex isn't flagging valid header contents."""
         headers_ok = {'foo': 'bar baz qux',
-                      'bar': '1',
+                      'bar': u'fbbq'.encode('utf8'),
                       'baz': '',
-                      'qux': str.encode(u'fbbq')}
+                      'qux': '1'}
         r = requests.get(httpbin('get'), headers=headers_ok)
         assert r.request.headers['foo'] == headers_ok['foo']
 
+    def test_header_value_not_str(self, httpbin):
+        """Ensure the header value is of type string or bytes as
+        per discussion in GH issue #3386
+        """
+        headers_int = {'foo': 3}
+        headers_dict = {'bar': {'foo':'bar'}}
+        headers_list = {'baz': ['foo', 'bar']}
+
+        # Test for int
+        with pytest.raises(InvalidHeader):
+            r = requests.get(httpbin('get'), headers=headers_int)
+        # Test for dict
+        with pytest.raises(InvalidHeader):
+            r = requests.get(httpbin('get'), headers=headers_dict)
+        # Test for list
+        with pytest.raises(InvalidHeader):
+            r = requests.get(httpbin('get'), headers=headers_list)
+
     def test_header_no_return_chars(self, httpbin):
         """Ensure that a header containing return character sequences raise an
         exception. Otherwise, multiple headers are created from single string.