diff --git a/index.js b/index.js
index fe1838b6..29a9efbf 100755
--- a/index.js
+++ b/index.js
@@ -220,11 +220,11 @@ Sharp.prototype.blur = function(radius) {
   } else if (typeof radius === 'boolean') {
     // Boolean argument: apply mild blur?
     this.options.blurRadius = radius ? -1 : 0;
-  } else if (typeof radius === 'number' && !Number.isNaN(radius) && (radius % 1 === 0)) {
+  } else if (typeof radius === 'number' && !Number.isNaN(radius) && (radius % 1 === 0) && radius >= 1) {
     // Numeric argument: specific radius
     this.options.blurRadius = radius;
   } else {
-    throw new Error('Invalid integral blur radius ' + radius);
+    throw new Error('Invalid blur radius ' + radius + ' (expected integer >= 1)');
   }
   return this;
 };
@@ -244,19 +244,27 @@ Sharp.prototype.sharpen = function(radius, flat, jagged) {
   } else if (typeof radius === 'boolean') {
     // Boolean argument: apply mild sharpen?
     this.options.sharpenRadius = radius ? -1 : 0;
-  } else if (typeof radius === 'number' && !Number.isNaN(radius) && (radius % 1 === 0)) {
+  } else if (typeof radius === 'number' && !Number.isNaN(radius) && (radius % 1 === 0) && radius >= 1) {
     // Numeric argument: specific radius
     this.options.sharpenRadius = radius;
-    if (typeof flat === 'number' && !Number.isNaN(flat)) {
-      // Control over flat areas
-      this.options.sharpenFlat = flat;
+    // Control over flat areas
+    if (typeof flat !== 'undefined' && flat !== null) {
+      if (typeof flat === 'number' && !Number.isNaN(flat) && flat >= 0) {
+        this.options.sharpenFlat = flat;
+      } else {
+        throw new Error('Invalid sharpen level for flat areas ' + flat + ' (expected >= 0)');
+      }
     }
-    if (typeof jagged === 'number' && !Number.isNaN(jagged)) {
-      // Control over jagged areas
-      this.options.sharpenJagged = jagged;
+    // Control over jagged areas
+    if (typeof jagged !== 'undefined' && jagged !== null) {
+      if (typeof jagged === 'number' && !Number.isNaN(jagged) && jagged >= 0) {
+        this.options.sharpenJagged = jagged;
+      } else {
+        throw new Error('Invalid sharpen level for jagged areas ' + jagged + ' (expected >= 0)');
+      }
     }
   } else {
-    throw new Error('Invalid integral sharpen radius ' + radius);
+    throw new Error('Invalid sharpen radius ' + radius + ' (expected integer >= 1)');
   }
   return this;
 };
diff --git a/test/unit/sharpen.js b/test/unit/sharpen.js
index 49cd7388..96bf024f 100755
--- a/test/unit/sharpen.js
+++ b/test/unit/sharpen.js
@@ -9,7 +9,20 @@ sharp.cache(0);
 
 describe('Sharpen', function() {
 
-  it('specific radius and levels 0.5, 2.5', function(done) {
+  it('specific radius 10', function(done) {
+    sharp(fixtures.inputJpg)
+      .resize(320, 240)
+      .sharpen(10)
+      .toFile(fixtures.path('output.sharpen-10.jpg'), function(err, info) {
+        if (err) throw err;
+        assert.strictEqual('jpeg', info.format);
+        assert.strictEqual(320, info.width);
+        assert.strictEqual(240, info.height);
+        done();
+      });
+  });
+
+  it('specific radius 3 and levels 0.5, 2.5', function(done) {
     sharp(fixtures.inputJpg)
       .resize(320, 240)
       .sharpen(3, 0.5, 2.5)
@@ -22,7 +35,7 @@ describe('Sharpen', function() {
       });
   });
 
-  it('specific radius 3 and levels 2, 4', function(done) {
+  it('specific radius 5 and levels 2, 4', function(done) {
     sharp(fixtures.inputJpg)
       .resize(320, 240)
       .sharpen(5, 2, 4)
@@ -59,6 +72,28 @@ describe('Sharpen', function() {
     done();
   });
 
+  it('invalid flat', function(done) {
+    var isValid = true;
+    try {
+      sharp(fixtures.inputJpg).sharpen(1, -1);
+    } catch (err) {
+      isValid = false;
+    }
+    assert.strictEqual(false, isValid);
+    done();
+  });
+
+  it('invalid jagged', function(done) {
+    var isValid = true;
+    try {
+      sharp(fixtures.inputJpg).sharpen(1, 1, -1);
+    } catch (err) {
+      isValid = false;
+    }
+    assert.strictEqual(false, isValid);
+    done();
+  });
+
   it('sharpened image is larger than non-sharpened', function(done) {
     sharp(fixtures.inputJpg)
       .resize(320, 240)