diff --git a/README.md b/README.md
index f0fa2b52..4e073dfe 100755
--- a/README.md
+++ b/README.md
@@ -378,7 +378,7 @@ Include all metadata (ICC, EXIF, XMP) from the input image in the output image.
 
 An advanced setting for the _zlib_ compression level of the lossless PNG output format. The default level is `6`.
 
-`compressionLevel` is a Number between -1 and 9.
+`compressionLevel` is a Number between 0 and 9.
 
 ### Output methods
 
diff --git a/index.js b/index.js
index 380199aa..ede6c40a 100755
--- a/index.js
+++ b/index.js
@@ -63,6 +63,7 @@ util.inherits(Sharp, stream.Duplex);
   Handle incoming chunk on Writable Stream
 */
 Sharp.prototype._write = function(chunk, encoding, callback) {
+  /*jslint unused: false */
   if (this.options.streamIn) {
     if (typeof chunk === 'object' || chunk instanceof Buffer) {
       if (typeof this.options.bufferIn === 'undefined') {
@@ -229,10 +230,10 @@ Sharp.prototype.quality = function(quality) {
 };
 
 Sharp.prototype.compressionLevel = function(compressionLevel) {
-  if (!Number.isNaN(compressionLevel) && compressionLevel >= -1 && compressionLevel <= 9) {
+  if (!Number.isNaN(compressionLevel) && compressionLevel >= 0 && compressionLevel <= 9) {
     this.options.compressionLevel = compressionLevel;
   } else {
-    throw new Error('Invalid compressionLevel (-1 to 9) ' + compressionLevel);
+    throw new Error('Invalid compressionLevel (0 to 9) ' + compressionLevel);
   }
   return this;
 };
diff --git a/tests/unit.js b/tests/unit.js
index 305b4c85..caabe4de 100755
--- a/tests/unit.js
+++ b/tests/unit.js
@@ -815,6 +815,26 @@ async.series([
       done();
     });
   },
+  // PNG compression level - valid
+  function(done) {
+    var isValid = false;
+    try {
+      sharp().compressionLevel(0);
+      isValid = true;
+    } catch (e) {}
+    assert(isValid);
+    done();
+  },
+  // PNG compression level - invalid
+  function(done) {
+    var isValid = false;
+    try {
+      sharp().compressionLevel(-1);
+      isValid = true;
+    } catch (e) {}
+    assert(!isValid);
+    done();
+  },
   // Verify internal counters
   function(done) {
     var counters = sharp.counters();