diff --git a/Sources/NIOHTTP2/HTTP2Error.swift b/Sources/NIOHTTP2/HTTP2Error.swift
index 402b5c56..b20330e5 100644
--- a/Sources/NIOHTTP2/HTTP2Error.swift
+++ b/Sources/NIOHTTP2/HTTP2Error.swift
@@ -200,6 +200,16 @@ public enum NIOHTTP2Errors {
             self.streamID = streamID
         }
     }
+
+    /// An attempt was made to send trailers without setting END_STREAM on them.
+    public struct TrailersWithoutEndStream: NIOHTTP2Error {
+        /// The affected stream ID.
+        public var streamID: HTTP2StreamID
+
+        public init(streamID: HTTP2StreamID) {
+            self.streamID = streamID
+        }
+    }
 }
 
 
diff --git a/Sources/NIOHTTP2/StreamStateMachine.swift b/Sources/NIOHTTP2/StreamStateMachine.swift
index 3401b70f..3f2e31c0 100644
--- a/Sources/NIOHTTP2/StreamStateMachine.swift
+++ b/Sources/NIOHTTP2/StreamStateMachine.swift
@@ -850,6 +850,12 @@ extension HTTP2StreamStateMachine {
     /// target final state.
     private mutating func processTrailers(_ headers: HPACKHeaders, isEndStreamSet endStream: Bool, targetState target: State, targetEffect effect: StreamStateChange?) -> StateMachineResultWithStreamEffect {
         // TODO(cory): Implement
+
+        // End stream must be set on trailers.
+        guard endStream else {
+            return StateMachineResultWithStreamEffect(result: .streamError(streamID: self.streamID, underlyingError: NIOHTTP2Errors.TrailersWithoutEndStream(streamID: self.streamID), type: .protocolError), effect: nil)
+        }
+
         self.state = target
         return StateMachineResultWithStreamEffect(result: .succeed, effect: effect)
     }
diff --git a/Tests/NIOHTTP2Tests/ConnectionStateMachineTests+XCTest.swift b/Tests/NIOHTTP2Tests/ConnectionStateMachineTests+XCTest.swift
index 770349c0..c11709b6 100644
--- a/Tests/NIOHTTP2Tests/ConnectionStateMachineTests+XCTest.swift
+++ b/Tests/NIOHTTP2Tests/ConnectionStateMachineTests+XCTest.swift
@@ -81,6 +81,8 @@ extension ConnectionStateMachineTests {
                 ("testDisablingPushPreventsPush", testDisablingPushPreventsPush),
                 ("testRatchetingGoawayEvenWhenFullyQueisced", testRatchetingGoawayEvenWhenFullyQueisced),
                 ("testRatchetingGoawayForBothPeersEvenWhenFullyQuiesced", testRatchetingGoawayForBothPeersEvenWhenFullyQuiesced),
+                ("testClientTrailersMustHaveEndStreamSet", testClientTrailersMustHaveEndStreamSet),
+                ("testServerTrailersMustHaveEndStreamSet", testServerTrailersMustHaveEndStreamSet),
            ]
    }
 }
diff --git a/Tests/NIOHTTP2Tests/ConnectionStateMachineTests.swift b/Tests/NIOHTTP2Tests/ConnectionStateMachineTests.swift
index 5c92ecf2..7ee6f0b0 100644
--- a/Tests/NIOHTTP2Tests/ConnectionStateMachineTests.swift
+++ b/Tests/NIOHTTP2Tests/ConnectionStateMachineTests.swift
@@ -1476,8 +1476,8 @@ class ConnectionStateMachineTests: XCTestCase {
 
         assertSucceeds(self.client.sendHeaders(streamID: streamOne, headers: ConnectionStateMachineTests.requestHeaders, isEndStreamSet: false))
         assertSucceeds(self.server.receiveHeaders(streamID: streamOne, headers: ConnectionStateMachineTests.requestHeaders, isEndStreamSet: false))
-        assertSucceeds(self.client.sendHeaders(streamID: streamOne, headers: ConnectionStateMachineTests.trailers, isEndStreamSet: false))
-        assertSucceeds(self.server.receiveHeaders(streamID: streamOne, headers: ConnectionStateMachineTests.trailers, isEndStreamSet: false))
+        assertSucceeds(self.client.sendHeaders(streamID: streamOne, headers: ConnectionStateMachineTests.trailers, isEndStreamSet: true))
+        assertSucceeds(self.server.receiveHeaders(streamID: streamOne, headers: ConnectionStateMachineTests.trailers, isEndStreamSet: true))
         assertSucceeds(self.server.sendHeaders(streamID: streamOne, headers: ConnectionStateMachineTests.responseHeaders, isEndStreamSet: false))
         assertSucceeds(self.client.receiveHeaders(streamID: streamOne, headers: ConnectionStateMachineTests.responseHeaders, isEndStreamSet: false))
         assertSucceeds(self.server.sendPushPromise(originalStreamID: streamOne, childStreamID: streamTwo, headers: ConnectionStateMachineTests.requestHeaders))
@@ -1721,5 +1721,37 @@ class ConnectionStateMachineTests: XCTestCase {
         assertGoawaySucceeds(self.client.sendGoaway(lastStreamID: .rootStream), droppingStreams: nil)
         assertGoawaySucceeds(self.server.receiveGoaway(lastStreamID: .rootStream), droppingStreams: nil)
     }
+
+    func testClientTrailersMustHaveEndStreamSet() {
+        let streamOne = HTTP2StreamID(1)
+
+        self.exchangePreamble()
+
+        // Client initiates a stream.
+        assertSucceeds(self.client.sendHeaders(streamID: streamOne, headers: ConnectionStateMachineTests.requestHeaders, isEndStreamSet: false))
+        assertSucceeds(self.server.receiveHeaders(streamID: streamOne, headers: ConnectionStateMachineTests.requestHeaders, isEndStreamSet: false))
+
+        // If the client attempts to send trailers without end stream, this fails with a stream error.
+        assertStreamError(type: .protocolError, self.client.sendHeaders(streamID: streamOne, headers: ConnectionStateMachineTests.trailers, isEndStreamSet: false))
+        assertStreamError(type: .protocolError, self.server.receiveHeaders(streamID: streamOne, headers: ConnectionStateMachineTests.trailers, isEndStreamSet: false))
+    }
+
+    func testServerTrailersMustHaveEndStreamSet() {
+        let streamOne = HTTP2StreamID(1)
+
+        self.exchangePreamble()
+
+        // Client initiates a stream.
+        assertSucceeds(self.client.sendHeaders(streamID: streamOne, headers: ConnectionStateMachineTests.requestHeaders, isEndStreamSet: true))
+        assertSucceeds(self.server.receiveHeaders(streamID: streamOne, headers: ConnectionStateMachineTests.requestHeaders, isEndStreamSet: true))
+
+        // Server sends back headers.
+        assertSucceeds(self.server.sendHeaders(streamID: streamOne, headers: ConnectionStateMachineTests.responseHeaders, isEndStreamSet: false))
+        assertSucceeds(self.client.receiveHeaders(streamID: streamOne, headers: ConnectionStateMachineTests.responseHeaders, isEndStreamSet: false))
+
+        // If the server attempts to send trailers without end stream, this fails with a stream error.
+        assertStreamError(type: .protocolError, self.server.sendHeaders(streamID: streamOne, headers: ConnectionStateMachineTests.trailers, isEndStreamSet: false))
+        assertStreamError(type: .protocolError, self.client.receiveHeaders(streamID: streamOne, headers: ConnectionStateMachineTests.trailers, isEndStreamSet: false))
+    }
 }