fix(ext/fetch): better error if no content-type

The streaming WASM support code inspects the Response object's
Content-Type header but if that was missing, it failed with a fairly
inscrutable "String.prototype.toLowerCase called on null or undefined"
exception. Now it raises a more legible "Invalid WebAssembly content
type" exception.

1 files changed, 3 insertions, 2 deletions

diff --git a/ext/fetch/26_fetch.js b/ext/fetch/26_fetch.js
index 2d7180691..edd14abf1 100644
--- a/ext/fetch/26_fetch.js
+++ b/ext/fetch/26_fetch.js
@@ -503,9 +503,10 @@
         // The spec is ambiguous here, see
         // https://github.com/WebAssembly/spec/issues/1138. The WPT tests
         // expect the raw value of the Content-Type attribute lowercased.
+        const contentType = res.headers.get("Content-Type");
         if (
-          StringPrototypeToLowerCase(res.headers.get("Content-Type")) !==
-            "application/wasm"
+          typeof contentType !== "string" ||
+          StringPrototypeToLowerCase(contentType) !== "application/wasm"
         ) {
           throw new TypeError("Invalid WebAssembly content type.");
         }