diff --git a/packages/core/src/RequestUtils.js b/packages/core/src/RequestUtils.js
index fd3ec4c8..5026df40 100644
--- a/packages/core/src/RequestUtils.js
+++ b/packages/core/src/RequestUtils.js
@@ -127,14 +127,16 @@ export function getQuery(url) {
 
 /**
  *
- * @param {Headers | [string, string][] | Record < string, string > | Object.<string, string | number> | HeadersInit} headers
+ * @param {HeadersInit | Object.<string, string |number>} headers
  * @returns {Object.<string, string>}
  */
 export const normalizeHeaders = (headers) => {
-	let entries = headers;
+	let entries;
 	if (headers instanceof Headers) {
 		entries = [...headers.entries()];
-	} else if (!Array.isArray(headers)) {
+	} else if (Array.isArray(headers)) {
+		entries = headers;
+	} else {
 		entries = Object.entries(headers);
 	}
 	return Object.fromEntries(
diff --git a/packages/core/src/Router.js b/packages/core/src/Router.js
index 469e54bc..3fb01ca1 100644
--- a/packages/core/src/Router.js
+++ b/packages/core/src/Router.js
@@ -93,33 +93,27 @@ function shouldSendAsObject(responseInput) {
 	return true;
 }
 
-function throwSpecExceptions(callLog) {
-	const headers = callLog.options.headers;
+/**
+ *
+ * @param {CallLog} callLog
+ */
+function throwSpecExceptions({ url, options: { headers, method, body } }) {
 	if (headers) {
 		Object.entries(headers).forEach(([key]) => {
 			if (/\s/.test(key)) {
-				throw new TypeError('no way - space in header');
+				throw new TypeError('Invalid name');
 			}
-
-			// if (/\s/.test(value)) {
-			// 	throw new TypeError('no way - space in header');
-			// }
 		});
 	}
-	if (/^[a-z]+\:\/\/[^:]+:[^@]+@/.test(callLog.url)) {
-		throw new TypeError('no way - contains credentials');
-	}
-
-	if (['navigate', 'websocket'].includes(callLog.options.mode)) {
-		throw new TypeError('no way - wrong mode');
+	const urlObject = new URL(url);
+	if (urlObject.username || urlObject.password) {
+		throw new TypeError(
+			`Request cannot be constructed from a URL that includes credentials: ${url}`,
+		);
 	}
 
-	console.log(callLog.options);
-	if (
-		['get', 'head'].includes(callLog.options.method) &&
-		callLog.options.body
-	) {
-		throw new TypeError('no way - wrong method for body');
+	if (['get', 'head'].includes(method) && body) {
+		throw new TypeError('Request with GET/HEAD method cannot have body.');
 	}
 }
 
diff --git a/packages/core/src/__tests__/CallHistory.test.js b/packages/core/src/__tests__/CallHistory.test.js
index d3ad9b1d..adacddf2 100644
--- a/packages/core/src/__tests__/CallHistory.test.js
+++ b/packages/core/src/__tests__/CallHistory.test.js
@@ -9,7 +9,7 @@ describe('CallHistory', () => {
 	});
 
 	const fetchTheseUrls = (...urls) =>
-		Promise.all(urls.map(fm.fetchHandler.bind(fm)));
+		Promise.all(urls.map((url) => fm.fetchHandler(url)));
 
 	describe('helper methods', () => {
 		describe('called()', () => {
@@ -208,8 +208,8 @@ describe('CallHistory', () => {
 				describe('boolean and named route filters', () => {
 					it('can retrieve calls matched by non-fallback routes', async () => {
 						fm.route('http://a.com/', 200).catch();
-
 						await fetchTheseUrls('http://a.com/', 'http://b.com/');
+
 						expectSingleUrl(true)('http://a.com/');
 						expectSingleUrl('matched')('http://a.com/');
 					});
@@ -298,12 +298,13 @@ describe('CallHistory', () => {
 							headers: { a: 'z' },
 						});
 						expect(filteredCalls.length).toEqual(1);
+
 						expect(filteredCalls[0]).toMatchObject(
 							expect.objectContaining({
 								url: 'http://a.com/',
-								options: {
+								options: expect.objectContaining({
 									headers: { a: 'z' },
-								},
+								}),
 							}),
 						);
 					});
@@ -326,9 +327,9 @@ describe('CallHistory', () => {
 						expect(filteredCalls[0]).toMatchObject(
 							expect.objectContaining({
 								url: 'http://b.com/',
-								options: {
+								options: expect.objectContaining({
 									headers: { a: 'z' },
-								},
+								}),
 							}),
 						);
 					});
@@ -347,9 +348,9 @@ describe('CallHistory', () => {
 						expect(filteredCalls[0]).toMatchObject(
 							expect.objectContaining({
 								url: 'http://a.com/',
-								options: {
+								options: expect.objectContaining({
 									headers: { a: 'z' },
-								},
+								}),
 							}),
 						);
 					});
diff --git a/packages/core/src/__tests__/spec-compliance.test.js b/packages/core/src/__tests__/spec-compliance.test.js
index c29f0232..994b82a6 100644
--- a/packages/core/src/__tests__/spec-compliance.test.js
+++ b/packages/core/src/__tests__/spec-compliance.test.js
@@ -12,38 +12,33 @@ describe('Spec compliance', () => {
 						'has space': 'ok',
 					},
 				}),
-			).rejects.toThrow(new TypeError('asdasdsa'));
-		});
-		it('reject on invalid header value', async () => {
-			await expect(
-				fetchMock.fetchHandler('http://a.com', {
-					headers: [['a', 'b', 'c']],
-				}),
-			).rejects.toThrow(new TypeError('asdasdsa'));
+			).rejects.toThrow(new TypeError('Invalid name'));
 		});
 		it('reject on url containing credentials', async () => {
 			await expect(
 				fetchMock.fetchHandler('http://user:password@a.com'),
-			).rejects.toThrow(new TypeError('asdasdsa'));
-		});
-		it('reject on invalid modes', async () => {
-			await expect(
-				fetchMock.fetchHandler('http://a.com', { mode: 'websocket' }),
-			).rejects.toThrow(new TypeError('asdasdsa'));
-			await expect(
-				fetchMock.fetchHandler('http://a.com', { mode: 'navigate' }),
-			).rejects.toThrow(new TypeError('asdasdsa'));
+			).rejects.toThrow(
+				new TypeError(
+					'Request cannot be constructed from a URL that includes credentials: http://user:password@a.com/',
+				),
+			);
 		});
 		it('reject if the request method is GET or HEAD and the body is non-null.', async () => {
 			await expect(
 				fetchMock.fetchHandler('http://a.com', { body: 'a' }),
-			).rejects.toThrow(new TypeError('asdasdsa'));
+			).rejects.toThrow(
+				new TypeError('Request with GET/HEAD method cannot have body.'),
+			);
 			await expect(
 				fetchMock.fetchHandler('http://a.com', { body: 'a', method: 'GET' }),
-			).rejects.toThrow(new TypeError('asdasdsa'));
+			).rejects.toThrow(
+				new TypeError('Request with GET/HEAD method cannot have body.'),
+			);
 			await expect(
 				fetchMock.fetchHandler('http://a.com', { body: 'a', method: 'HEAD' }),
-			).rejects.toThrow(new TypeError('asdasdsa'));
+			).rejects.toThrow(
+				new TypeError('Request with GET/HEAD method cannot have body.'),
+			);
 		});
 	});
 });
diff --git a/packages/core/types/RequestUtils.d.ts b/packages/core/types/RequestUtils.d.ts
index 12db62ed..20560f1f 100644
--- a/packages/core/types/RequestUtils.d.ts
+++ b/packages/core/types/RequestUtils.d.ts
@@ -3,9 +3,9 @@ export function createCallLogFromUrlAndOptions(url: string | object, options: Re
 export function createCallLogFromRequest(request: Request, options: RequestInit): Promise<CallLog>;
 export function getPath(url: string): string;
 export function getQuery(url: string): string;
-export function normalizeHeaders(headers: Headers | [string, string][] | Record<string, string> | {
+export function normalizeHeaders(headers: HeadersInit | {
     [x: string]: string | number;
-} | HeadersInit): {
+}): {
     [x: string]: string;
 };
 export type DerivedRequestOptions = {