diff --git a/src/Microsoft.AspNetCore.Sockets.Common/Internal/Formatters/ServerSentEventsMessageParser.cs b/src/Microsoft.AspNetCore.Sockets.Common/Internal/Formatters/ServerSentEventsMessageParser.cs
index b786986dbc..a7086dd648 100644
--- a/src/Microsoft.AspNetCore.Sockets.Common/Internal/Formatters/ServerSentEventsMessageParser.cs
+++ b/src/Microsoft.AspNetCore.Sockets.Common/Internal/Formatters/ServerSentEventsMessageParser.cs
@@ -23,6 +23,8 @@ namespace Microsoft.AspNetCore.Sockets.Internal.Formatters
         private static byte[] _sseLineEnding = Encoding.UTF8.GetBytes("\r\n");
         private static byte[] _newLine = Encoding.UTF8.GetBytes(Environment.NewLine);
 
+        private readonly static int _messageTypeLineLength = "data: X\r\n".Length;
+
         private InternalParseState _internalParserState = InternalParseState.ReadMessageType;
         private List<byte[]> _data = new List<byte[]>();
         private MessageType _messageType = MessageType.Text;
@@ -94,7 +96,10 @@ namespace Microsoft.AspNetCore.Sockets.Internal.Formatters
                 switch (_internalParserState)
                 {
                     case InternalParseState.ReadMessageType:
-                        _messageType = GetMessageType(line);
+                        EnsureStartsWithDataPrefix(line);
+
+
+                        _messageType = ParseMessageType(line);
 
                         _internalParserState = InternalParseState.ReadMessagePayload;
 
@@ -102,6 +107,8 @@ namespace Microsoft.AspNetCore.Sockets.Internal.Formatters
                         consumed = lineEnd;
                         break;
                     case InternalParseState.ReadMessagePayload:
+                        EnsureStartsWithDataPrefix(line);
+
                         // Slice away the 'data: '
                         var payloadLength = line.Length - (_dataPrefix.Length + _sseLineEnding.Length);
                         var newData = line.Slice(_dataPrefix.Length, payloadLength).ToArray();
@@ -198,9 +205,12 @@ namespace Microsoft.AspNetCore.Sockets.Internal.Formatters
             return line.Length == _sseLineEnding.Length && line.SequenceEqual(_sseLineEnding);
         }
 
-        private MessageType GetMessageType(ReadOnlySpan<byte> line)
+        private MessageType ParseMessageType(ReadOnlySpan<byte> line)
         {
-            EnsureStartsWithDataPrefix(line);
+            if (line.Length != _messageTypeLineLength)
+            {
+                throw new FormatException("Expected a data format message of the form 'data: <MesssageType>'");
+            }
 
             // Skip the "data: " part of the line
             var type = line[_dataPrefix.Length];
diff --git a/test/Microsoft.AspNetCore.Sockets.Common.Tests/Internal/Formatters/ServerSentEventsParserTests.cs b/test/Microsoft.AspNetCore.Sockets.Common.Tests/Internal/Formatters/ServerSentEventsParserTests.cs
index 302608b9a2..2ecf01c0c8 100644
--- a/test/Microsoft.AspNetCore.Sockets.Common.Tests/Internal/Formatters/ServerSentEventsParserTests.cs
+++ b/test/Microsoft.AspNetCore.Sockets.Common.Tests/Internal/Formatters/ServerSentEventsParserTests.cs
@@ -48,19 +48,20 @@ namespace Microsoft.AspNetCore.Sockets.Common.Tests.Internal.Formatters
         [InlineData("data: X\r\n", "Unknown message type: 'X'")]
         [InlineData("data: T\n", "Unexpected '\n' in message. A '\n' character can only be used as part of the newline sequence '\r\n'")]
         [InlineData("data: X\r\n\r\n", "Unknown message type: 'X'")]
-        [InlineData("data: Not the message type\r\n\r\n", "Unknown message type: 'N'")]
+        [InlineData("data: Not the message type\r\n\r\n", "Expected a data format message of the form 'data: <MesssageType>'")]
         [InlineData("data: T\r\ndata: Hello, World\r\r\n\n", "There was an error in the frame format")]
-        [InlineData("data: Not the message type\r\r\n", "Unknown message type: 'N'")]
+        [InlineData("data: Not the message type\r\r\n", "Expected a data format message of the form 'data: <MesssageType>'")]
         [InlineData("data: T\r\ndata: Hello, World\n\n", "Unexpected '\n' in message. A '\n' character can only be used as part of the newline sequence '\r\n'")]
         [InlineData("data: T\r\nfoo: Hello, World\r\n\r\n", "Expected the message prefix 'data: '")]
         [InlineData("foo: T\r\ndata: Hello, World\r\n\r\n", "Expected the message prefix 'data: '")]
         [InlineData("food: T\r\ndata: Hello, World\r\n\r\n", "Expected the message prefix 'data: '")]
         [InlineData("data: T\r\ndata: Hello, World\r\n\n", "There was an error in the frame format")]
         [InlineData("data: T\r\ndata: Hello\n, World\r\n\r\n", "Unexpected '\n' in message. A '\n' character can only be used as part of the newline sequence '\r\n'")]
-        [InlineData("data: data: \r\n", "Unknown message type: 'd'")]
+        [InlineData("data: data: \r\n", "Expected a data format message of the form 'data: <MesssageType>'")]
         [InlineData("data: T\r\ndata: Hello, World\r\n\r\\", "Expected a \\r\\n frame ending")]
         [InlineData("data: T\r\ndata: Major\r\ndata:  Key\rndata:  Alert\r\n\r\\", "Expected a \\r\\n frame ending")]
         [InlineData("data: T\r\ndata: Major\r\ndata:  Key\r\ndata:  Alert\r\n\r\\", "Expected a \\r\\n frame ending")]
+        [InlineData("data: This is not a message type\r\n", "Expected a data format message of the form 'data: <MesssageType>'")]
         [InlineData("data: B\r\n SGVsbG8sIFdvcmxk\r\n\r\n", "Expected the message prefix 'data: '")]
         public void ParseSSEMessageFailureCases(string encodedMessage, string expectedExceptionMessage)
         {
@@ -155,18 +156,20 @@ namespace Microsoft.AspNetCore.Sockets.Common.Tests.Internal.Formatters
         [InlineData("data: ", "X\r\n", "Unknown message type: 'X'")]
         [InlineData("data: T", "\n", "Unexpected '\n' in message. A '\n' character can only be used as part of the newline sequence '\r\n'")]
         [InlineData("data: ", "X\r\n\r\n", "Unknown message type: 'X'")]
-        [InlineData("data: ", "Not the message type\r\n\r\n", "Unknown message type: 'N'")]
+        [InlineData("data: ", "Not the message type\r\n\r\n", "Expected a data format message of the form 'data: <MesssageType>'")]
         [InlineData("data: T\r\n", "data: Hello, World\r\r\n\n", "There was an error in the frame format")]
-        [InlineData("data:", " Not the message type\r\r\n", "Unknown message type: 'N'")]
+        [InlineData("data:", " Not the message type\r\r\n", "Expected a data format message of the form 'data: <MesssageType>'")]
         [InlineData("data: T\r\n", "data: Hello, World\n\n", "Unexpected '\n' in message. A '\n' character can only be used as part of the newline sequence '\r\n'")]
         [InlineData("data: T\r\nf", "oo: Hello, World\r\n\r\n", "Expected the message prefix 'data: '")]
         [InlineData("foo", ": T\r\ndata: Hello, World\r\n\r\n", "Expected the message prefix 'data: '")]
         [InlineData("food:", " T\r\ndata: Hello, World\r\n\r\n", "Expected the message prefix 'data: '")]
         [InlineData("data: T\r\ndata: Hello, W", "orld\r\n\n", "There was an error in the frame format")]
         [InlineData("data: T\r\nda", "ta: Hello\n, World\r\n\r\n", "Unexpected '\n' in message. A '\n' character can only be used as part of the newline sequence '\r\n'")]
-        [InlineData("data:", " data: \r\n", "Unknown message type: 'd'")]
+        [InlineData("data:", " data: \r\n", "Expected a data format message of the form 'data: <MesssageType>'")]
         [InlineData("data: ", "T\r\ndata: Major\r\ndata:  Key\r\ndata:  Alert\r\n\r\\", "Expected a \\r\\n frame ending")]
+        [InlineData("data: ", "This is not a message type\r\n", "Expected a data format message of the form 'data: <MesssageType>'")]
         [InlineData("data: B\r\ndata: SGVs", "bG8sIFdvcmxk\r\n\n\n", "There was an error in the frame format")]
+        [InlineData("data: T", "his is not a message type\r\n", "Expected a data format message of the form 'data: <MesssageType>'")]
         public async Task ParseMessageAcrossMultipleReadsFailure(string encodedMessagePart1, string encodedMessagePart2, string expectedMessage)
         {
             using (var pipeFactory = new PipeFactory())