diff --git a/Octokit.Tests.Integration/Clients/PullRequestsClientTests.cs b/Octokit.Tests.Integration/Clients/PullRequestsClientTests.cs
index d021995f..94e6387b 100644
--- a/Octokit.Tests.Integration/Clients/PullRequestsClientTests.cs
+++ b/Octokit.Tests.Integration/Clients/PullRequestsClientTests.cs
@@ -238,8 +238,7 @@ public class PullRequestsClientTests : IDisposable
         var merge = new MergePullRequest { Sha = fakeSha };
         var ex = await Assert.ThrowsAsync<PullRequestMismatchException>(() => _fixture.Merge(Helper.UserName, _context.RepositoryName, pullRequest.Number, merge));
 
-        //merge exceptions don't inherit from ApiException so ApiError is not available
-        Assert.True(ex.Message.StartsWith("The merge operation specified a SHA which didn't match"));
+        Assert.True(ex.Message.StartsWith("Head branch was modified"));
     }
 
     [IntegrationTest]
@@ -257,8 +256,7 @@ public class PullRequestsClientTests : IDisposable
         var merge = new MergePullRequest { Sha = pullRequest.Head.Sha };
         var ex = await Assert.ThrowsAsync<PullRequestNotMergeableException>(() => _fixture.Merge(Helper.UserName, _context.RepositoryName, pullRequest.Number, merge));
 
-        //merge exceptions don't inherit from ApiException so ApiError is not available
-        Assert.True(ex.Message.Equals("The pull request is not in a mergeable state"));
+        Assert.True(ex.Message.Equals("Pull Request is not mergeable"));
     }
 
     [IntegrationTest]
diff --git a/Octokit/Clients/PullRequestsClient.cs b/Octokit/Clients/PullRequestsClient.cs
index b650be46..b73da1fb 100644
--- a/Octokit/Clients/PullRequestsClient.cs
+++ b/Octokit/Clients/PullRequestsClient.cs
@@ -124,12 +124,12 @@ namespace Octokit
             {
                 if (ex.StatusCode == HttpStatusCode.MethodNotAllowed)
                 {
-                    throw new PullRequestNotMergeableException();
+                    throw new PullRequestNotMergeableException(ex.HttpResponse);
                 }
 
                 if (ex.StatusCode == HttpStatusCode.Conflict)
                 {
-                    throw new PullRequestMismatchException();
+                    throw new PullRequestMismatchException(ex.HttpResponse);
                 }
 
                 throw;
diff --git a/Octokit/Exceptions/PullRequestMismatchException.cs b/Octokit/Exceptions/PullRequestMismatchException.cs
index 8a77d6dc..f16504b5 100644
--- a/Octokit/Exceptions/PullRequestMismatchException.cs
+++ b/Octokit/Exceptions/PullRequestMismatchException.cs
@@ -1,4 +1,7 @@
 using System;
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+using System.Net;
 using System.Runtime.Serialization;
 
 namespace Octokit
@@ -10,28 +13,39 @@ namespace Octokit
 #if !NETFX_CORE
     [Serializable]
 #endif
-    public class PullRequestMismatchException : Exception
+    [SuppressMessage("Microsoft.Design", "CA1032:ImplementStandardExceptionConstructors",
+        Justification = "These exceptions are specific to the GitHub API and not general purpose exceptions")]
+    public class PullRequestMismatchException : ApiException
     {
-        public PullRequestMismatchException()
-            : base("The merge operation specified a SHA which didn't match " +
-                   "the SHA of the pull request's HEAD")
+        /// <summary>
+        /// Constructs an instace of <see cref="Octokit.PullRequestMismatchException"/>.
+        /// </summary>
+        /// <param name="response"></param>
+        public PullRequestMismatchException(IResponse response) : this(response, null)
         {
         }
 
-        public PullRequestMismatchException(string message)
-            : base(message)
+        /// <summary>
+        /// Constructs an instance of <see cref="Octokit.PullRequestMismatchException"/>.
+        /// </summary>
+        /// <param name="response">The HTTP payload from the server</param>
+        /// <param name="innerException">The inner exception</param>
+        public PullRequestMismatchException(IResponse response, Exception innerException)
+            : base(response, innerException)
         {
+            Debug.Assert(response != null && response.StatusCode == HttpStatusCode.Conflict,
+                "PullRequestMismatchException created with the wrong HTTP status code");
         }
 
-
-        public PullRequestMismatchException(string message, Exception innerException)
-            : base(message, innerException)
+        public override string Message
         {
+            //https://developer.github.com/v3/pulls/#response-if-sha-was-provided-and-pull-request-head-did-not-match
+            get { return ApiErrorMessageSafe ?? "Head branch was modified. Review and try the merge again."; }
         }
 
 #if !NETFX_CORE
         /// <summary>
-        /// Constructs an instance of PullRequestNotMergeableException.
+        /// Constructs an instance of <see cref="Octokit.PullRequestNotMergeableException"/>.
         /// </summary>
         /// <param name="info">
         /// The <see cref="SerializationInfo"/> that holds the
@@ -47,6 +61,4 @@ namespace Octokit
         }
 #endif
     }
-
-
 }
diff --git a/Octokit/Exceptions/PullRequestNotMergeableException.cs b/Octokit/Exceptions/PullRequestNotMergeableException.cs
index e24c2d36..ca4143dc 100644
--- a/Octokit/Exceptions/PullRequestNotMergeableException.cs
+++ b/Octokit/Exceptions/PullRequestNotMergeableException.cs
@@ -1,4 +1,7 @@
 using System;
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+using System.Net;
 using System.Runtime.Serialization;
 
 namespace Octokit
@@ -10,26 +13,39 @@ namespace Octokit
 #if !NETFX_CORE
     [Serializable]
 #endif
-    public class PullRequestNotMergeableException : Exception
+    [SuppressMessage("Microsoft.Design", "CA1032:ImplementStandardExceptionConstructors",
+        Justification = "These exceptions are specific to the GitHub API and not general purpose exceptions")]
+    public class PullRequestNotMergeableException : ApiException
     {
-        public PullRequestNotMergeableException()
-            : base("The pull request is not in a mergeable state")
+        /// <summary>
+        /// Constructs an instance of the <see cref="Octokit.PullRequestNotMergeableException"/> class.
+        /// </summary>
+        /// <param name="response">The HTTP payload from the server</param>
+        public PullRequestNotMergeableException(IResponse response) : this(response, null)
         {
         }
 
-        public PullRequestNotMergeableException(string message)
-            : base(message)
+        /// <summary>
+        /// Constructs an instance of the <see cref="Octokit.PullRequestNotMergeableException"/> class.
+        /// </summary>
+        /// <param name="response">The HTTP payload from the server</param>
+        /// <param name="innerException">The inner exception</param>
+        public PullRequestNotMergeableException(IResponse response, Exception innerException)
+            : base(response, innerException)
         {
+            Debug.Assert(response != null && response.StatusCode == HttpStatusCode.MethodNotAllowed,
+            "PullRequestNotMergeableException created with the wrong HTTP status code");
         }
 
-        public PullRequestNotMergeableException(string message, Exception innerException)
-            : base(message, innerException)
+        public override string Message
         {
+            //https://developer.github.com/v3/pulls/#response-if-merge-cannot-be-performed
+            get { return ApiErrorMessageSafe ?? "Pull Request is not mergeable"; }
         }
 
 #if !NETFX_CORE
         /// <summary>
-        /// Constructs an instance of PullRequestNotMergeableException.
+        /// Constructs an instance of <see cref="Octokit.PullRequestNotMergeableException"/>.
         /// </summary>
         /// <param name="info">
         /// The <see cref="SerializationInfo"/> that holds the