diff --git a/index.js b/index.js
index 16d079e..8c80a2b 100644
--- a/index.js
+++ b/index.js
@@ -38,7 +38,12 @@ const replaceValueSymbols = (valueString, replacements) => {
 };
 
 const getDefinition = (atRule, existingDefinitions, requiredDefinitions) => {
-  const [/* match */, name, middle, value, end] = matchValueDefinition.exec(atRule.params);
+  const matches = matchValueDefinition.exec(atRule.params);
+  if (!matches) {
+    return null;
+  }
+
+  const [/* match */, name, middle, value, end] = matches;
   const valueWithReplacements = replaceValueSymbols(value, existingDefinitions);
 
   if (!requiredDefinitions) {
@@ -110,6 +115,9 @@ const walk = async (requiredDefinitions, walkFile, root, result) => {
     }
 
     const newDefinitions = getDefinition(atRule, existingDefinitions, requiredDefinitions);
+    if (!newDefinitions) {
+      result.warn(`Invalid value definition: ${atRule.params}`);
+    }
     return Object.assign(existingDefinitions, newDefinitions);
   };
 
diff --git a/index.test.js b/index.test.js
index 9e1ab6a..579766a 100644
--- a/index.test.js
+++ b/index.test.js
@@ -51,7 +51,7 @@ test('should remove exports if noEmitExports is true', async (t) => {
   await run(t, '@value red blue;', '', { noEmitExports: true });
 });
 
-test('gives an error when there is no semicolon between lines', async (t) => {
+test('gives a warning when there is no semicolon between lines', async (t) => {
   const input = '@value red blue\n@value green yellow';
   const processor = postcss([plugin]);
   const result = await processor.process(input, { from: undefined });
@@ -61,13 +61,23 @@ test('gives an error when there is no semicolon between lines', async (t) => {
   t.expect(warnings[0].text).toBe('Invalid value definition: red blue\n@value green yellow');
 });
 
+test('gives a warning when @value definition is invalid', async (t) => {
+  const input = '@value oops:;';
+  const processor = postcss([plugin]);
+  const result = await processor.process(input, { from: undefined });
+  const warnings = result.warnings();
+
+  t.expect(warnings.length).toBe(1);
+  t.expect(warnings[0].text).toBe('Invalid value definition: oops:');
+});
+
 test('gives an error when path to imported file is wrong', async (t) => {
   const input = '@value red from "./non-existent-file.css"';
   const processor = postcss([plugin]);
   await t.expect(processor.process(input, parserOpts)).rejects.toThrow("Can't resolve './non-existent-file.css'");
 });
 
-test('gives an error when @value statement is invalid', async (t) => {
+test('gives an error when @value import statement is invalid', async (t) => {
   const input = '@value , from "./colors.css"';
   const processor = postcss([plugin]);
   await t.expect(processor.process(input, parserOpts)).rejects.toThrow('@value statement "" is invalid!');