Return undefined loc if declaration missing source

packages/core/integration-tests/test/integration/postcss-plugins-decl/index.css

@@ -0,0 +1,2 @@
+.index {
+}

packages/core/integration-tests/test/integration/postcss-plugins-decl/postcss.config.js

@@ -0,0 +1,11 @@
+module.exports = {
+  "plugins": [
+    {
+      postcssPlugin: 'PLUGIN NAME',
+      Rule: (rule, { Declaration }) => {
+        const decl = new Declaration({ prop: 'background-image', value: 'url("data:image/gif;base64,quotes")' })
+        rule.append(decl)
+      },
+    }
+  ]
+}

packages/core/integration-tests/test/postcss.js

@@ -251,6 +251,16 @@ describe('postcss', () => {
     assert.equal(run1(), run2());
   });
 
+  it('should support transforming declarations with missing source', async () => {
+    await bundle(
+      path.join(__dirname, '/integration/postcss-plugins-decl/index.css'),
+    );
+
+    let css = await outputFS.readFile(path.join(distDir, 'index.css'), 'utf8');
+
+    assert(css.includes('url("data:image/gif;base64,quotes")'));
+  });
+
   it('should support postcss composes imports', async () => {
     let b = await bundle(
       path.join(__dirname, '/integration/postcss-composes/index.js'),

packages/transformers/css/src/CSSTransformer.js

@@ -143,12 +143,15 @@ export default (new Transformer({
             !node.nodes[0].value.startsWith('#') // IE's `behavior: url(#default#VML)`
           ) {
             let url = asset.addURLDependency(node.nodes[0].value, {
-              loc: createLoc(
-                nullthrows(decl.source.start),
-                node.nodes[0].value,
-                0,
-                node.nodes[0].sourceIndex,
-              ),
+              loc:
+                decl.source &&
+                decl.source.start &&
+                createLoc(
+                  decl.source.start,
+                  node.nodes[0].value,
+                  0,
+                  node.nodes[0].sourceIndex,
+                ),
             });
             isDeclDirty = node.nodes[0].value !== url;
             node.nodes[0].value = url;