Improve error message
diff --git a/src/csharp/Grpc.Core.Api/ChannelCredentials.cs b/src/csharp/Grpc.Core.Api/ChannelCredentials.cs
index ad5b687..29bb39f 100644
--- a/src/csharp/Grpc.Core.Api/ChannelCredentials.cs
+++ b/src/csharp/Grpc.Core.Api/ChannelCredentials.cs
@@ -104,7 +104,7 @@
 
                 if (!channelCredentials.IsComposable)
                 {
-                    throw new ArgumentException(string.Format("{0} credentials can't be used with CallCredentials. Secure credentials like SslCredentials must be used.", channelCredentials.GetType().Name));
+                    throw new ArgumentException(string.Format("{0} can't be composed with supplied CallCredentials. Secure credentials like SslCredentials must be used.", channelCredentials.GetType().Name));
                 }
             }
 
diff --git a/src/csharp/Grpc.Core.Tests/ChannelCredentialsTest.cs b/src/csharp/Grpc.Core.Tests/ChannelCredentialsTest.cs
index 8919a88..9576e05 100644
--- a/src/csharp/Grpc.Core.Tests/ChannelCredentialsTest.cs
+++ b/src/csharp/Grpc.Core.Tests/ChannelCredentialsTest.cs
@@ -41,7 +41,7 @@
 
             // forbid composing non-composable
             var ex = Assert.Throws(typeof(ArgumentException), () => ChannelCredentials.Create(new FakeChannelCredentials(false), new FakeCallCredentials()));
-            Assert.AreEqual("FakeChannelCredentials credentials can't be used with CallCredentials. Secure credentials like SslCredentials must be used.", ex.Message);
+            Assert.AreEqual("FakeChannelCredentials can't be composed with supplied CallCredentials. Secure credentials like SslCredentials must be used.", ex.Message);
         }
 
         [Test]