diff --git a/cssesc.js b/cssesc.js
index 1c0928e..2e28abc 100644
--- a/cssesc.js
+++ b/cssesc.js
@@ -72,7 +72,10 @@ var cssesc = function cssesc(string, options) {
 	}
 
 	if (isIdentifier) {
-		if (/^-[-\d]/.test(output)) {
+		if (output == '-') {
+			// A lone `-` is not a valid identifier; escape it.
+			output = '\\-';
+		} else if (/^-[-\d]/.test(output)) {
 			output = '\\-' + output.slice(1);
 		} else if (/\d/.test(firstChar)) {
 			output = '\\3' + firstChar + ' ' + output.slice(1);
diff --git a/src/cssesc.js b/src/cssesc.js
index adeac60..f0f3c76 100644
--- a/src/cssesc.js
+++ b/src/cssesc.js
@@ -83,7 +83,10 @@ const cssesc = (string, options) => {
 	}
 
 	if (isIdentifier) {
-		if (/^-[-\d]/.test(output)) {
+		if (output == '-') {
+			// A lone `-` is not a valid identifier; escape it.
+			output = '\\-';
+		} else if (/^-[-\d]/.test(output)) {
 			output = '\\-' + output.slice(1);
 		} else if (/\d/.test(firstChar)) {
 			output = '\\3' + firstChar + ' ' + output.slice(1);
diff --git a/tests/tests.js b/tests/tests.js
index 6cabb6d..0e105f2 100644
--- a/tests/tests.js
+++ b/tests/tests.js
@@ -25,6 +25,16 @@ describe('common usage', () => {
 			'\\--foo',
 			'--foo with `isIdentifier: true`'
 		);
+		assert.equal(
+			cssesc('-', { 'isIdentifier': false }),
+			'-',
+			'lone - with `isIdentifier: false`'
+		);
+		assert.equal(
+			cssesc('-', { 'isIdentifier': true }),
+			'\\-',
+			'lone - with `isIdentifier: true`'
+		);
 		assert.equal(
 			cssesc('-0foo', { 'isIdentifier': false }),
 			'-0foo',