Fix gradient parser

lib/hacks/gradient.js

@@ -55,6 +55,10 @@ class Gradient extends Value {
         continue
       }
 
+      if (param.length === 0) {
+        continue
+      }
+
       let color = parser.stringify(param[0])
       if (param[1] && param[1].type === 'word') {
         pos = param[1].value
@@ -75,7 +79,7 @@ class Gradient extends Value {
 
       let div = param[param.length - 1]
       params[i] = [{ type: 'word', value: stop }]
-      if (div.type === 'div' && div.value === ',') {
+      if (div && div.type === 'div' && div.value === ',') {
         item = params[i].push(div)
       }
       result.push(item)
@@ -184,6 +188,9 @@ class Gradient extends Value {
    * Replace old direction to new
    */
   newDirection(params) {
+    if (!params[0]) {
+      return params
+    }
     if (params[0].value === 'to') {
       return params
     }
@@ -297,7 +304,7 @@ class Gradient extends Value {
   oldDirection(params) {
     let div = this.cloneDiv(params[0])
 
-    if (params[0][0].value !== 'to') {
+    if (!params[0][0] || params[0][0].value !== 'to') {
       return params.unshift([
         { type: 'word', value: Gradient.oldDirections.bottom },
         div
@@ -328,6 +335,9 @@ class Gradient extends Value {
     if (this.name !== 'linear-gradient') {
       return false
     }
+    if (nodes.length === 0) {
+      return false
+    }
     if (nodes[0] && nodes[0].value.includes('deg')) {
       return false
     }

test/autoprefixer.test.js

@@ -905,6 +905,24 @@ test('skips old webkit gradient for CSS variables', () => {
   equal(result.css.includes('-webkit-linear-gradient('), true)
 })
 
+test('does not throw on empty or malformed gradient arguments', () => {
+  let inputs = [
+    'a { background-image: linear-gradient() }',
+    'a { background-image: radial-gradient() }',
+    'a { background-image: repeating-linear-gradient() }',
+    'a { background-image: repeating-radial-gradient() }',
+    'a { background-image: linear-gradient(red,) }',
+    'a { background-image: linear-gradient(,red) }',
+    'a { background-image: linear-gradient(red,,blue) }',
+    'a { background-image: linear-gradient(to right, red,) }'
+  ]
+  for (let input of inputs) {
+    // Would throw an uncaught TypeError before the empty-argument guards
+    let result = postcss([gradienter]).process(input, { from: undefined })
+    type(result.css, 'string')
+  }
+})
+
 test('warns on old flexbox display', () => {
   let result = postcss([flexboxer]).process('a{ display: box; }')
   equal(result.css, 'a{ display: box; }')