diff --git a/src/MagicString/index.js b/src/MagicString/index.js
index 80f004f..807eb95 100644
--- a/src/MagicString/index.js
+++ b/src/MagicString/index.js
@@ -19,6 +19,10 @@ MagicString.prototype = {
 	},
 
 	append: function ( content ) {
+		if ( typeof content !== 'string' ) {
+			throw new TypeError( 'appended content must be a string' );
+		}
+
 		this.str += content;
 		return this;
 	},
@@ -165,6 +169,10 @@ MagicString.prototype = {
 	},
 
 	insert: function ( index, content ) {
+		if ( typeof content !== 'string' ) {
+			throw new TypeError( 'inserted content must be a string' );
+		}
+
 		if ( index === 0 ) {
 			this.prepend( content );
 		} else if ( index === this.original.length ) {
@@ -224,6 +232,10 @@ MagicString.prototype = {
 	},
 
 	replace: function ( start, end, content ) {
+		if ( typeof content !== 'string' ) {
+			throw new TypeError( 'replacement content must be a string' );
+		}
+
 		var firstChar, lastChar, d;
 
 		firstChar = this.locate( start );
diff --git a/test/index.js b/test/index.js
index fc8ba65..36eeb70 100644
--- a/test/index.js
+++ b/test/index.js
@@ -30,6 +30,14 @@ describe( 'MagicString', function () {
 			var s = new MagicString( 'abcdefghijkl' );
 			assert.strictEqual( s.append( 'xyz' ), s );
 		});
+
+		it( 'should throw when given non-string content', function () {
+			var s = new MagicString( '' );
+			assert.throws(
+				function () { s.append( [] ); },
+				TypeError
+			);
+		});
 	});
 
 	describe( 'clone', function () {
@@ -263,6 +271,14 @@ describe( 'MagicString', function () {
 			assert.equal( s.insert(1, '1').toString(), 'a1b' );
 			assert.equal( s.insert(1, '2').toString(), 'a12b' );
 		});
+
+		it( 'should throw when given non-string content', function () {
+			var s = new MagicString( '' );
+			assert.throws(
+				function () { s.insert( 0, [] ); },
+				TypeError
+			);
+		});
 	});
 
 	describe( 'locate', function () {
@@ -466,6 +482,14 @@ describe( 'MagicString', function () {
 			var s = new MagicString( 'abcdefghijkl' );
 			assert.strictEqual( s.replace( 3, 4, 'D' ), s );
 		});
+
+		it( 'should throw when given non-string content', function () {
+			var s = new MagicString( '' );
+			assert.throws(
+				function () { s.replace( 0, 1, [] ); },
+				TypeError
+			);
+		});
 	});
 
 	describe( 'slice', function () {