diff --git a/src/WireMock.Net/Matchers/Request/RequestMessageCookieMatcher.cs b/src/WireMock.Net/Matchers/Request/RequestMessageCookieMatcher.cs
index e0e07c64..56664493 100644
--- a/src/WireMock.Net/Matchers/Request/RequestMessageCookieMatcher.cs
+++ b/src/WireMock.Net/Matchers/Request/RequestMessageCookieMatcher.cs
@@ -86,17 +86,19 @@ namespace WireMock.Matchers.Request
 
         private double IsMatch(RequestMessage requestMessage)
         {
-            if (Funcs != null)
-                return MatchScores.ToScore(requestMessage.Cookies != null && Funcs.Any(cf => cf(requestMessage.Cookies)));
-
             if (requestMessage.Cookies == null)
                 return MatchScores.Mismatch;
 
+            if (Funcs != null)
+                return MatchScores.ToScore(Funcs.Any(f => f(requestMessage.Cookies)));
+
+            if (Matchers == null)
+                return MatchScores.Mismatch;
+
             if (!requestMessage.Cookies.ContainsKey(Name))
                 return MatchScores.Mismatch;
 
             string value = requestMessage.Cookies[Name];
-
             return Matchers.Max(m => m.IsMatch(value));
         }
     }
diff --git a/src/WireMock.Net/Matchers/Request/RequestMessageHeaderMatcher.cs b/src/WireMock.Net/Matchers/Request/RequestMessageHeaderMatcher.cs
index 24c3c7da..7127f324 100644
--- a/src/WireMock.Net/Matchers/Request/RequestMessageHeaderMatcher.cs
+++ b/src/WireMock.Net/Matchers/Request/RequestMessageHeaderMatcher.cs
@@ -86,18 +86,20 @@ namespace WireMock.Matchers.Request
 
         private double IsMatch(RequestMessage requestMessage)
         {
-            if (Funcs != null)
-                return MatchScores.ToScore(requestMessage.Headers != null && Funcs.Any(hf => hf(requestMessage.Headers)));
-
             if (requestMessage.Headers == null)
                 return MatchScores.Mismatch;
 
+            if (Funcs != null)
+                return MatchScores.ToScore(Funcs.Any(f => f(requestMessage.Headers)));
+
+            if (Matchers == null)
+                return MatchScores.Mismatch;
+
             if (!requestMessage.Headers.ContainsKey(Name))
                 return MatchScores.Mismatch;
 
-            string headerValue = requestMessage.Headers[Name];
-
-            return Matchers.Max(m => m.IsMatch(headerValue));
+            string value = requestMessage.Headers[Name];
+            return Matchers.Max(m => m.IsMatch(value));
         }
     }
 }
\ No newline at end of file
diff --git a/src/WireMock.Net/Matchers/Request/RequestMessageParamMatcher.cs b/src/WireMock.Net/Matchers/Request/RequestMessageParamMatcher.cs
index 2f215c27..942b123b 100644
--- a/src/WireMock.Net/Matchers/Request/RequestMessageParamMatcher.cs
+++ b/src/WireMock.Net/Matchers/Request/RequestMessageParamMatcher.cs
@@ -36,10 +36,9 @@ namespace WireMock.Matchers.Request
         /// <param name="values">
         /// The values.
         /// </param>
-        public RequestMessageParamMatcher([NotNull] string key, [NotNull] IEnumerable<string> values)
+        public RequestMessageParamMatcher([NotNull] string key, [CanBeNull] IEnumerable<string> values)
         {
             Check.NotNull(key, nameof(key));
-            Check.NotNull(values, nameof(values));
 
             Key = key;
             Values = values;
diff --git a/src/WireMock.Net/RequestBuilders/IBodyRequestBuilder.cs b/src/WireMock.Net/RequestBuilders/IBodyRequestBuilder.cs
index 5aa52039..c979b91e 100644
--- a/src/WireMock.Net/RequestBuilders/IBodyRequestBuilder.cs
+++ b/src/WireMock.Net/RequestBuilders/IBodyRequestBuilder.cs
@@ -35,13 +35,13 @@ namespace WireMock.RequestBuilders
         /// </summary>
         /// <param name="body">The body string function.</param>
         /// <returns>The <see cref="IRequestBuilder"/>.</returns>
-        IRequestBuilder WithBody(Func<string, bool> body);
+        IRequestBuilder WithBody([NotNull] Func<string, bool> body);
 
         /// <summary>
         /// The with body byte[] func.
         /// </summary>
         /// <param name="body">The body byte[] function.</param>
         /// <returns>The <see cref="IRequestBuilder"/>.</returns>
-        IRequestBuilder WithBody(Func<byte[], bool> body);
+        IRequestBuilder WithBody([NotNull] Func<byte[], bool> body);
     }
 }
\ No newline at end of file
diff --git a/src/WireMock.Net/RequestBuilders/IParamsRequestBuilder.cs b/src/WireMock.Net/RequestBuilders/IParamsRequestBuilder.cs
index fb7ba015..590eefa8 100644
--- a/src/WireMock.Net/RequestBuilders/IParamsRequestBuilder.cs
+++ b/src/WireMock.Net/RequestBuilders/IParamsRequestBuilder.cs
@@ -16,7 +16,7 @@ namespace WireMock.RequestBuilders
         /// <param name="key">The key.</param>
         /// <param name="values">The values.</param>
         /// <returns>The <see cref="IRequestBuilder"/>.</returns>
-        IRequestBuilder WithParam([NotNull] string key, params string[] values);
+        IRequestBuilder WithParam([NotNull] string key, [CanBeNull] params string[] values);
 
         /// <summary>
         /// The with parameters.
diff --git a/src/WireMock.Net/RequestBuilders/Request.cs b/src/WireMock.Net/RequestBuilders/Request.cs
index cee17dda..144a2adf 100644
--- a/src/WireMock.Net/RequestBuilders/Request.cs
+++ b/src/WireMock.Net/RequestBuilders/Request.cs
@@ -5,6 +5,7 @@ using System.Linq;
 using WireMock.Matchers;
 using WireMock.Matchers.Request;
 using WireMock.Util;
+using WireMock.Validation;
 
 namespace WireMock.RequestBuilders
 {
@@ -60,6 +61,8 @@ namespace WireMock.RequestBuilders
         /// <returns>The <see cref="IRequestBuilder"/>.</returns>
         public IRequestBuilder WithPath(params IMatcher[] matchers)
         {
+            Check.NotEmpty(matchers, nameof(matchers));
+
             _requestMatchers.Add(new RequestMessagePathMatcher(matchers));
             return this;
         }
@@ -71,6 +74,8 @@ namespace WireMock.RequestBuilders
         /// <returns>The <see cref="IRequestBuilder"/>.</returns>
         public IRequestBuilder WithPath(params string[] paths)
         {
+            Check.NotEmpty(paths, nameof(paths));
+
             _requestMatchers.Add(new RequestMessagePathMatcher(paths));
             return this;
         }
@@ -82,6 +87,8 @@ namespace WireMock.RequestBuilders
         /// <returns>The <see cref="IRequestBuilder"/>.</returns>
         public IRequestBuilder WithPath(params Func<string, bool>[] funcs)
         {
+            Check.NotEmpty(funcs, nameof(funcs));
+
             _requestMatchers.Add(new RequestMessagePathMatcher(funcs));
             return this;
         }
@@ -89,11 +96,13 @@ namespace WireMock.RequestBuilders
         /// <summary>
         /// The with url.
         /// </summary>
-        /// <param name="matcher">The matcher.</param>
+        /// <param name="matchers">The matchers.</param>
         /// <returns>The <see cref="IRequestBuilder"/>.</returns>
-        public IRequestBuilder WithUrl(params IMatcher[] matcher)
+        public IRequestBuilder WithUrl(params IMatcher[] matchers)
         {
-            _requestMatchers.Add(new RequestMessageUrlMatcher(matcher));
+            Check.NotEmpty(matchers, nameof(matchers));
+
+            _requestMatchers.Add(new RequestMessageUrlMatcher(matchers));
             return this;
         }
 
@@ -104,6 +113,8 @@ namespace WireMock.RequestBuilders
         /// <returns>The <see cref="IRequestBuilder"/>.</returns>
         public IRequestBuilder WithUrl(params string[] urls)
         {
+            Check.NotEmpty(urls, nameof(urls));
+
             _requestMatchers.Add(new RequestMessageUrlMatcher(urls));
             return this;
         }
@@ -115,6 +126,8 @@ namespace WireMock.RequestBuilders
         /// <returns>The <see cref="IRequestBuilder"/>.</returns>
         public IRequestBuilder WithUrl(params Func<string, bool>[] funcs)
         {
+            Check.NotEmpty(funcs, nameof(funcs));
+
             _requestMatchers.Add(new RequestMessageUrlMatcher(funcs));
             return this;
         }
@@ -201,6 +214,8 @@ namespace WireMock.RequestBuilders
         /// <returns>The <see cref="IRequestBuilder"/>.</returns>
         public IRequestBuilder UsingVerb(params string[] verbs)
         {
+            Check.NotEmpty(verbs, nameof(verbs));
+
             _requestMatchers.Add(new RequestMessageMethodMatcher(verbs));
             return this;
         }
@@ -240,6 +255,8 @@ namespace WireMock.RequestBuilders
         /// <returns>The <see cref="IRequestBuilder"/>.</returns>
         public IRequestBuilder WithBody(Func<string, bool> func)
         {
+            Check.NotNull(func, nameof(func));
+
             _requestMatchers.Add(new RequestMessageBodyMatcher(func));
             return this;
         }
@@ -253,6 +270,8 @@ namespace WireMock.RequestBuilders
         /// <returns>The <see cref="IRequestBuilder"/>.</returns>
         public IRequestBuilder WithBody(Func<byte[], bool> func)
         {
+            Check.NotNull(func, nameof(func));
+
             _requestMatchers.Add(new RequestMessageBodyMatcher(func));
             return this;
         }
@@ -264,6 +283,8 @@ namespace WireMock.RequestBuilders
         /// <returns>The <see cref="IRequestBuilder" />.</returns>
         public IRequestBuilder WithBody(IMatcher matcher)
         {
+            Check.NotNull(matcher, nameof(matcher));
+
             _requestMatchers.Add(new RequestMessageBodyMatcher(matcher));
             return this;
         }
@@ -280,6 +301,8 @@ namespace WireMock.RequestBuilders
         /// <returns>The <see cref="IRequestBuilder"/>.</returns>
         public IRequestBuilder WithParam(string key, params string[] values)
         {
+            Check.NotNull(key, nameof(key));
+
             _requestMatchers.Add(new RequestMessageParamMatcher(key, values));
             return this;
         }
@@ -291,6 +314,8 @@ namespace WireMock.RequestBuilders
         /// <returns>The <see cref="IRequestBuilder"/>.</returns>
         public IRequestBuilder WithParam(params Func<IDictionary<string, WireMockList<string>>, bool>[] funcs)
         {
+            Check.NotEmpty(funcs, nameof(funcs));
+
             _requestMatchers.Add(new RequestMessageParamMatcher(funcs));
             return this;
         }
@@ -304,6 +329,9 @@ namespace WireMock.RequestBuilders
         /// <returns>The <see cref="IRequestBuilder"/>.</returns>
         public IRequestBuilder WithHeader(string name, string pattern, bool ignoreCase = true)
         {
+            Check.NotNull(name, nameof(name));
+            Check.NotNull(pattern, nameof(pattern));
+
             _requestMatchers.Add(new RequestMessageHeaderMatcher(name, pattern, ignoreCase));
             return this;
         }
@@ -316,6 +344,9 @@ namespace WireMock.RequestBuilders
         /// <returns>The <see cref="IRequestBuilder"/>.</returns>
         public IRequestBuilder WithHeader(string name, params IMatcher[] matchers)
         {
+            Check.NotNull(name, nameof(name));
+            Check.NotEmpty(matchers, nameof(matchers));
+
             _requestMatchers.Add(new RequestMessageHeaderMatcher(name, matchers));
             return this;
         }
@@ -327,6 +358,8 @@ namespace WireMock.RequestBuilders
         /// <returns>The <see cref="IRequestBuilder"/>.</returns>
         public IRequestBuilder WithHeader(params Func<IDictionary<string, string>, bool>[] funcs)
         {
+            Check.NotEmpty(funcs, nameof(funcs));
+
             _requestMatchers.Add(new RequestMessageHeaderMatcher(funcs));
             return this;
         }
@@ -352,6 +385,8 @@ namespace WireMock.RequestBuilders
         /// <returns>The <see cref="IRequestBuilder"/>.</returns>
         public IRequestBuilder WithCookie(string name, params IMatcher[] matchers)
         {
+            Check.NotEmpty(matchers, nameof(matchers));
+
             _requestMatchers.Add(new RequestMessageCookieMatcher(name, matchers));
             return this;
         }
@@ -363,6 +398,8 @@ namespace WireMock.RequestBuilders
         /// <returns>The <see cref="IRequestBuilder"/>.</returns>
         public IRequestBuilder WithCookie(params Func<IDictionary<string, string>, bool>[] funcs)
         {
+            Check.NotEmpty(funcs, nameof(funcs));
+
             _requestMatchers.Add(new RequestMessageCookieMatcher(funcs));
             return this;
         }
diff --git a/src/WireMock.Net/Server/FluentMockServer.Admin.cs b/src/WireMock.Net/Server/FluentMockServer.Admin.cs
index 52fc2045..55c2094f 100644
--- a/src/WireMock.Net/Server/FluentMockServer.Admin.cs
+++ b/src/WireMock.Net/Server/FluentMockServer.Admin.cs
@@ -85,7 +85,7 @@ namespace WireMock.Server
                 .WithGuid(guid)
                 .RespondWith(responseBuilder);
 
-            return new ResponseMessage { Body = "Mapping updated" };
+            return new ResponseMessage { Body = "Mapping added or updated" };
         }
 
         private ResponseMessage MappingDelete(RequestMessage requestMessage)