diff --git a/src/UriTemplateString/Spec/Operator.cs b/src/UriTemplateString/Spec/Operator.cs
index 4f17636..a67bd30 100644
--- a/src/UriTemplateString/Spec/Operator.cs
+++ b/src/UriTemplateString/Spec/Operator.cs
@@ -79,6 +79,12 @@ public static Operator FromString(string operatorCharacter)
                     return Operator.PathParameter;
                 case "&":
                     return Operator.QueryContinuation;
+                case "!":
+                case "=":
+                case ",":
+                case "@":
+                case "|":
+                    throw new NotImplementedException($"The operator character {operatorCharacter} is reserved for future extensions and not currently supported");
                 default:
                     throw new ArgumentOutOfRangeException(nameof(operatorCharacter));
             }
diff --git a/test/UriTemplateString.Tests/InvalidUriTemplateFixture.cs b/test/UriTemplateString.Tests/InvalidUriTemplateFixture.cs
new file mode 100644
index 0000000..c0f17d1
--- /dev/null
+++ b/test/UriTemplateString.Tests/InvalidUriTemplateFixture.cs
@@ -0,0 +1,26 @@
+using System;
+using Xunit;
+
+namespace UriTemplateString.Tests
+{
+    public class InvalidUriTemplateFixture
+    {
+        [Fact]
+        public void Should_throw_when_created_from_empty_string()
+        {
+            Assert.Throws<ArgumentException>(() => new UriTemplateString(string.Empty));
+        }
+
+        [Fact]
+        public void Should_throw_when_created_from_template_with_unclosed_expression()
+        {
+            Assert.Throws<ArgumentException>(() => new UriTemplateString("{/unclosed"));
+        }
+
+        [Fact]
+        public void Should_throw_when_created_from_template_with_unsupported_expression_operator()
+        {
+            Assert.Throws<ArgumentException>(() => new UriTemplateString("{'unclosed}"));
+        }
+    }
+}
\ No newline at end of file
diff --git a/test/UriTemplateString.Tests/OperatorFixture.cs b/test/UriTemplateString.Tests/OperatorFixture.cs
new file mode 100644
index 0000000..5182cfe
--- /dev/null
+++ b/test/UriTemplateString.Tests/OperatorFixture.cs
@@ -0,0 +1,26 @@
+using System;
+using UriTemplateString.Spec;
+using Xunit;
+
+namespace UriTemplateString.Tests
+{
+    public class OperatorFixture
+    {
+        [Fact]
+        public void Should_throw_when_constructed_with_unsupported_operator_character()
+        {
+            Assert.Throws<ArgumentOutOfRangeException>(() => Operator.FromString("_"));
+        }
+
+        [Theory]
+        [InlineData("=")]
+        [InlineData(",")]
+        [InlineData("!")]
+        [InlineData("@")]
+        [InlineData("|")]
+        public void Should_throw_when_constructed_with_operator_reserved_for_future_use(string op)
+        {
+            Assert.Throws<NotImplementedException>(() => Operator.FromString(op));
+        }
+    }
+}
\ No newline at end of file
diff --git a/test/UriTemplateString.Tests/UriTemplateString.Tests.csproj b/test/UriTemplateString.Tests/UriTemplateString.Tests.csproj
index 09ccc55..21b8cae 100644
--- a/test/UriTemplateString.Tests/UriTemplateString.Tests.csproj
+++ b/test/UriTemplateString.Tests/UriTemplateString.Tests.csproj
@@ -46,6 +46,8 @@
     <Reference Include="System.Xml" />
   </ItemGroup>
   <ItemGroup>
+    <Compile Include="InvalidUriTemplateFixture.cs" />
+    <Compile Include="OperatorFixture.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="UriTemplateStringArithmeticsFixture.cs" />
     <Compile Include="UriTemplateStringOperatorsFixture.cs" />