diff --git a/lib/headers.js b/lib/headers.js
index 2d8c96e..25e49ff 100644
--- a/lib/headers.js
+++ b/lib/headers.js
@@ -5,6 +5,30 @@
  */
 const utils = require('./utils');
 
+const REFERER_HEADER_NAMES = ['referer', 'referrer'];
+
+/**
+ * Get a header value from the headers object.
+ * Because headers can be set in multiple ways, their names can be uppercased and lowercased.
+ *
+ * @param {Object} headers
+ * @param {string} name
+ * @returns
+ */
+function getHeaderValue(headers, name) {
+    const lowerName = name.toLowerCase();
+
+    return headers[
+        Object.keys(headers).find((key) => {
+            const lowerKey = key.toLowerCase();
+            return (
+                lowerKey === lowerName ||
+                (REFERER_HEADER_NAMES.includes(lowerKey) && REFERER_HEADER_NAMES.includes(lowerName))
+            );
+        })
+    ];
+}
+
 /**
  * Creates a Headers object that implements both Express.js style access
  * and the Web API Headers interface
@@ -18,24 +42,17 @@ function createHeaders(headers = {}) {
             // Handle Headers interface methods
             switch (prop) {
                 case 'get':
-                    return (name) => {
-                        const lowerName = name.toLowerCase();
-                        // Special case for referer/referrer
-                        if (lowerName === 'referer' || lowerName === 'referrer') {
-                            return target.referrer || target.referer;
-                        }
-                        return target[lowerName];
-                    };
+                    return (name) => getHeaderValue(target, name);
                 case 'getAll':
                     return (name) => {
-                        const value = target[name.toLowerCase()];
+                        const value = getHeaderValue(target, name);
                         if (!value) {
                             return [];
                         }
                         return Array.isArray(value) ? value : [value];
                     };
                 case 'has':
-                    return (name) => name.toLowerCase() in target;
+                    return (name) => getHeaderValue(target, name) !== undefined;
                 case 'set':
                     return (name, value) => {
                         // eslint-disable-next-line no-param-reassign
@@ -82,7 +99,7 @@ function createHeaders(headers = {}) {
                         }
                     );
                 default:
-                    return target[typeof prop === 'string' ? prop.toLowerCase() : prop];
+                    return typeof prop === 'string' ? getHeaderValue(target, prop) : target[prop];
             }
         },
         set(target, prop, value) {
@@ -117,4 +134,4 @@ function createHeaders(headers = {}) {
     });
 }
 
-module.exports = { createHeaders };
+module.exports = { createHeaders, getHeaderValue };
diff --git a/lib/mockRequest.js b/lib/mockRequest.js
index 34de081..6098eae 100644
--- a/lib/mockRequest.js
+++ b/lib/mockRequest.js
@@ -34,7 +34,7 @@ const accepts = require('accepts');
 const parseRange = require('range-parser');
 let { EventEmitter } = require('events');
 const querystring = require('querystring');
-const { createHeaders } = require('./headers');
+const { createHeaders, getHeaderValue } = require('./headers');
 
 const standardRequestOptions = [
     'method',
@@ -130,7 +130,7 @@ function createRequest(options = {}) {
      * @api public
      */
     mockRequest.getHeader = function getHeader(name) {
-        return mockRequest.headers.get(name);
+        return getHeaderValue(mockRequest.headers, name);
     };
     mockRequest.header = mockRequest.getHeader;
     mockRequest.get = mockRequest.getHeader;
diff --git a/test/lib/mockRequest.spec.ts b/test/lib/mockRequest.spec.ts
index 3ef173e..de1f1e8 100644
--- a/test/lib/mockRequest.spec.ts
+++ b/test/lib/mockRequest.spec.ts
@@ -304,18 +304,46 @@ describe('mockRequest', () => {
     });
 
     describe('.get()/.header()', () => {
-        it('should return header, when set', () => {
+        it('should return header, when set in constructor', () => {
             const options = {
                 headers: {
-                    key: 'value'
+                    'Content-type': 'value'
                 }
             };
+
             const request = mockRequest.createRequest(options);
-            expect(request.get('key')).to.equal('value');
-            expect(request.header('key')).to.equal('value');
-            expect(request.headers.get('key')).to.equal('value');
-            expect(request.getHeader('key')).to.equal('value');
-            expect(request.headers.key).to.equal('value');
+            expect(request.get('Content-type')).to.equal('value');
+            expect(request.header('Content-type')).to.equal('value');
+            expect(request.headers.get('Content-type')).to.equal('value');
+            expect(request.getHeader('Content-type')).to.equal('value');
+            expect(request.headers['Content-type']).to.equal('value');
+        });
+
+        it('should return header, when set explicitly', () => {
+            const request = mockRequest.createRequest();
+
+            request.headers['Content-type'] = 'value';
+
+            expect(request.get('Content-type')).to.equal('value');
+            expect(request.header('Content-type')).to.equal('value');
+            expect(request.headers.get('Content-type')).to.equal('value');
+            expect(request.getHeader('Content-type')).to.equal('value');
+            expect(request.headers['Content-type']).to.equal('value');
+        });
+
+        it('should return header, when set as an object (deprecated)', () => {
+            const request = mockRequest.createRequest();
+
+            // setting headers as an object is officially supported by Express
+            // @ts-expect-error
+            request.headers = { 'Content-type': 'value' };
+
+            expect(request.get('Content-type')).to.equal('value');
+            expect(request.header('Content-type')).to.equal('value');
+            expect(request.getHeader('Content-type')).to.equal('value');
+            expect(request.headers['Content-type']).to.equal('value');
+
+            // request.headers.get() is not working in this case
         });
 
         it('should return referer, when request as referrer', () => {