diff --git a/lib/core/request.js b/lib/core/request.js
index c6d887d3fd5..936e11d8db1 100644
--- a/lib/core/request.js
+++ b/lib/core/request.js
@@ -410,145 +410,74 @@ function processHeader (request, key, val, skipAppend = false) {
     return
   }
 
-  const mayBeLowerCasedKey = headerNameLowerCasedRecord[key]
-
-  if (mayBeLowerCasedKey !== undefined) {
-    switch (mayBeLowerCasedKey) {
-      case 'host': {
-        if (request.host !== null) break
-        if (headerCharRegex.exec(val) !== null) {
-          throw new InvalidArgumentError(`invalid ${key} header`)
-        }
-        // Consumed by Client
-        request.host = val
-        return
-      }
-      case 'content-length': {
-        if (request.contentLength !== null) break
-        request.contentLength = parseInt(val, 10)
-        if (!Number.isFinite(request.contentLength)) {
-          throw new InvalidArgumentError('invalid content-length header')
-        }
-        return
-      }
-      case 'content-type': {
-        if (request.contentType !== null) break
-        request.contentType = val
-        if (skipAppend) request.headers[key] = processHeaderValue(key, val, skipAppend)
-        else request.headers += processHeaderValue(key, val)
-        return
-      }
-      case 'transfer-encoding': {
-        throw new InvalidArgumentError('invalid transfer-encoding header')
-      }
-      case 'connection': {
-        const value = typeof val === 'string' ? val.toLowerCase() : null
-        if (value !== 'close' && value !== 'keep-alive') {
-          throw new InvalidArgumentError('invalid connection header')
-        } else if (value === 'close') {
-          request.reset = true
-        }
-        return
-      }
-      case 'keep-alive': {
-        throw new InvalidArgumentError('invalid keep-alive header')
-      }
-      case 'upgrade': {
-        throw new InvalidArgumentError('invalid upgrade header')
-      }
-      case 'expect': {
-        throw new NotSupportedError('expect header not supported')
+  let headerName = headerNameLowerCasedRecord[key]
+
+  if (headerName === undefined) {
+    headerName = key.toLowerCase()
+    if (!util.isValidHTTPToken(headerName)) {
+      throw new InvalidArgumentError('invalid header key')
+    }
+  }
+
+  switch (headerName) {
+    case 'host': {
+      if (request.host !== null) break
+      if (headerCharRegex.exec(val) !== null) {
+        throw new InvalidArgumentError(`invalid ${key} header`)
       }
+      // Consumed by Client
+      request.host = val
+      return
     }
-    if (Array.isArray(val)) {
-      for (let i = 0; i < val.length; i++) {
-        if (skipAppend) {
-          if (request.headers[key]) request.headers[key] += `,${processHeaderValue(key, val[i], skipAppend)}`
-          else request.headers[key] = processHeaderValue(key, val[i], skipAppend)
-        } else {
-          request.headers += processHeaderValue(key, val[i])
-        }
+    case 'content-length': {
+      if (request.contentLength !== null) break
+      request.contentLength = parseInt(val, 10)
+      if (!Number.isFinite(request.contentLength)) {
+        throw new InvalidArgumentError('invalid content-length header')
       }
-    } else {
+      return
+    }
+    case 'content-type': {
+      if (request.contentType !== null) break
+      request.contentType = val
       if (skipAppend) request.headers[key] = processHeaderValue(key, val, skipAppend)
       else request.headers += processHeaderValue(key, val)
+      return
+    }
+    case 'transfer-encoding': {
+      throw new InvalidArgumentError('invalid transfer-encoding header')
+    }
+    case 'connection': {
+      const value = typeof val === 'string' ? val.toLowerCase() : null
+      if (value !== 'close' && value !== 'keep-alive') {
+        throw new InvalidArgumentError('invalid connection header')
+      } else if (value === 'close') {
+        request.reset = true
+      }
+      return
+    }
+    case 'keep-alive': {
+      throw new InvalidArgumentError('invalid keep-alive header')
+    }
+    case 'upgrade': {
+      throw new InvalidArgumentError('invalid upgrade header')
+    }
+    case 'expect': {
+      throw new NotSupportedError('expect header not supported')
     }
-    return
   }
-
-  if (
-    request.host === null &&
-    key.length === 4 &&
-    key.toLowerCase() === 'host'
-  ) {
-    if (headerCharRegex.exec(val) !== null) {
-      throw new InvalidArgumentError(`invalid ${key} header`)
-    }
-    // Consumed by Client
-    request.host = val
-  } else if (
-    request.contentLength === null &&
-    key.length === 14 &&
-    key.toLowerCase() === 'content-length'
-  ) {
-    request.contentLength = parseInt(val, 10)
-    if (!Number.isFinite(request.contentLength)) {
-      throw new InvalidArgumentError('invalid content-length header')
-    }
-  } else if (
-    request.contentType === null &&
-    key.length === 12 &&
-    key.toLowerCase() === 'content-type'
-  ) {
-    request.contentType = val
-    if (skipAppend) request.headers[key] = processHeaderValue(key, val, skipAppend)
-    else request.headers += processHeaderValue(key, val)
-  } else if (
-    key.length === 17 &&
-    key.toLowerCase() === 'transfer-encoding'
-  ) {
-    throw new InvalidArgumentError('invalid transfer-encoding header')
-  } else if (
-    key.length === 10 &&
-    key.toLowerCase() === 'connection'
-  ) {
-    const value = typeof val === 'string' ? val.toLowerCase() : null
-    if (value !== 'close' && value !== 'keep-alive') {
-      throw new InvalidArgumentError('invalid connection header')
-    } else if (value === 'close') {
-      request.reset = true
-    }
-  } else if (
-    key.length === 10 &&
-    key.toLowerCase() === 'keep-alive'
-  ) {
-    throw new InvalidArgumentError('invalid keep-alive header')
-  } else if (
-    key.length === 7 &&
-    key.toLowerCase() === 'upgrade'
-  ) {
-    throw new InvalidArgumentError('invalid upgrade header')
-  } else if (
-    key.length === 6 &&
-    key.toLowerCase() === 'expect'
-  ) {
-    throw new NotSupportedError('expect header not supported')
-  } else if (!util.isValidHTTPToken(key)) {
-    throw new InvalidArgumentError('invalid header key')
-  } else {
-    if (Array.isArray(val)) {
-      for (let i = 0; i < val.length; i++) {
-        if (skipAppend) {
-          if (request.headers[key]) request.headers[key] += `,${processHeaderValue(key, val[i], skipAppend)}`
-          else request.headers[key] = processHeaderValue(key, val[i], skipAppend)
-        } else {
-          request.headers += processHeaderValue(key, val[i])
-        }
+  if (Array.isArray(val)) {
+    for (let i = 0; i < val.length; i++) {
+      if (skipAppend) {
+        if (request.headers[key]) request.headers[key] += `,${processHeaderValue(key, val[i], skipAppend)}`
+        else request.headers[key] = processHeaderValue(key, val[i], skipAppend)
+      } else {
+        request.headers += processHeaderValue(key, val[i])
       }
-    } else {
-      if (skipAppend) request.headers[key] = processHeaderValue(key, val, skipAppend)
-      else request.headers += processHeaderValue(key, val)
     }
+  } else {
+    if (skipAppend) request.headers[key] = processHeaderValue(key, val, skipAppend)
+    else request.headers += processHeaderValue(key, val)
   }
 }