Added check for restricted XML characters.

RestrictedChar ::= [#x1-#x8] | [#xB-#xC] | [#xE-#x1F] | [#x7F-#x84] | [#x86-#x9F].

DocX/HelperFunctions.cs

         public const string DOCUMENT_DOCUMENTTYPE = "application/vnd.openxmlformats-officedocument.wordprocessingml.document.main+xml";
         public const string TEMPLATE_DOCUMENTTYPE = "application/vnd.openxmlformats-officedocument.wordprocessingml.template.main+xml";
 
-            public static bool IsNullOrWhiteSpace(this string value)
+		/// <summary>
+		///  List of restricted character in xml: [#x1-#x8] | [#xB-#xC] | [#xE-#x1F] | [#x7F-#x84] | [#x86-#x9F]
+		///  See:  https://www.w3.org/TR/xml11/#sec-xml11
+		/// </summary>
+		public static readonly char[] RestrictedXmlChar = new char[] {
+			'\x1','\x2','\x3','\x4','\x5','\x6','\x7','\x8','\xb','\xc','\xe','\xf',
+			'\x10','\x11','\x12','\x13','\x14','\x15','\x16','\x17','\x18','\x19','\x1a','\x1b','\x1c','\x1e','\x1f',
+			'\x7f','\x80','\x81','\x82','\x83','\x84','\x86','\x87','\x88','\x89','\x8a','\x8b','\x8c','\x8d','\x8e','\x8f',
+			'\x90','\x91','\x92','\x93','\x94','\x95','\x96','\x97','\x98','\x99','\x9a','\x9b','\x9c','\x9d','\x9e','\x9f'
+		};
+
+		public static bool IsNullOrWhiteSpace(this string value)
             {
                 if (value == null) return true;
                 return string.IsNullOrEmpty(value.Trim());
                         break;
 
                     default:
-                        sb.Append(c);
+						// Check the character against restricted list:
+						// RestrictedChar	   ::=   	[#x1-#x8] | [#xB-#xC] | [#xE-#x1F] | [#x7F-#x84] | [#x86-#x9F]
+						// See https://www.w3.org/TR/xml11/#sec-xml11 
+						if( RestrictedXmlChar.Contains( c ) )
+						{
+							// skip the character
+						}
+						else
+							sb.Append(c);
                         break;
                 }
                 

DocX/bin/Release/DocX.XML

             The custom property to display.
             </summary>
         </member>
+        <member name="F:Novacode.HelperFunctions.RestrictedXmlChar">
+            <summary>
+             List of restricted character in xml: [#x1-#x8] | [#xB-#xC] | [#xE-#x1F] | [#x7F-#x84] | [#x86-#x9F]
+             See:  https://www.w3.org/TR/xml11/#sec-xml11
+            </summary>
+        </member>
         <member name="M:Novacode.HelperFunctions.ContainsEveryChildOf(System.Xml.Linq.XElement,System.Xml.Linq.XElement,Novacode.MatchFormattingOptions)">
             <summary>
             Checks whether 'toCheck' has all children that 'desired' has and values of 'val' attributes are the same

UnitTests/DocXUnitTests.cs

             }
         }
 
+		[Test]
+		public void Test_InvalidCharacter()
+		{
+			using( var output = File.Open( Path.Combine( _directoryWithFiles, "InvalidCharacters.docx" ), FileMode.Create ) )
+			{
+				using( var doc = DocX.Create( output ) )
+				{
+					doc.InsertParagraph( "\b" );
+					Exception ex = null;
+					try
+					{
+						doc.Save();
+					}
+					catch( Exception e )
+					{
+						ex = e;
+					}
+					Assert.IsTrue( ex == null );
+				}
+			}
+		}
+
 		/// <summary>
 		/// TextRemove should not remove empty paragraphs in case the paragraph is alone in the cell.
 		/// In the rest cases empty paragraph may be removed.