diff --git a/fetch.js b/fetch.js
index 018bbb36..e31e4307 100644
--- a/fetch.js
+++ b/fetch.js
@@ -44,7 +44,7 @@ function normalizeName(name) {
   if (typeof name !== 'string') {
     name = String(name)
   }
-  if (/[^a-z0-9\-#$%&'*+.^_`|~]/i.test(name) || name === '') {
+  if (/[^a-z0-9\-#$%&'*+.^_`|~!]/i.test(name) || name === '') {
     throw new TypeError('Invalid character in header field name')
   }
   return name.toLowerCase()