fix: throw if an invalid fetch function is given

src/client.ts

@@ -317,13 +317,13 @@ export class Client {
 			this.queryContentFromRef(options.ref);
 		}
 
-		if (options.fetch) {
+		if (typeof options.fetch === "function") {
 			this.fetchFn = options.fetch;
 		} else if (typeof globalThis.fetch === "function") {
 			this.fetchFn = globalThis.fetch;
 		} else {
 			throw new Error(
-				"A fetch implementation was not provided. In environments where fetch is not available (including Node.js), a fetch implementation must be provided via a polyfill or the `fetch` option."
+				"A valid fetch implementation was not provided. In environments where fetch is not available (including Node.js), a fetch implementation must be provided via a polyfill or the `fetch` option."
 			);
 		}
 	}

test/client.test.ts

@@ -52,6 +52,25 @@ test("constructor throws if fetch is unavailable", t => {
 	});
 });
 
+test("constructor throws if provided fetch is not a function", t => {
+	const endpoint = prismic.getEndpoint("qwerty");
+	const fetch = "not a function";
+
+	t.throws(
+		() =>
+			prismic.createClient(endpoint, {
+				// We wouldn't normally test for input types since TypeScript handles
+				// that for us at build time, but we want to provide a nicer DX for
+				// non-TypeScript users.
+				// @ts-expect-error - We are purposly providing an invalid type to test if it throws.
+				fetch
+			}),
+		{
+			message: /fetch implementation was not provided/
+		}
+	);
+});
+
 test("uses globalThis.fetch if available", t => {
 	const endpoint = prismic.getEndpoint("qwerty");