diff --git a/protocol_grpc.go b/protocol_grpc.go
index 4b5e4492..a102dcdd 100644
--- a/protocol_grpc.go
+++ b/protocol_grpc.go
@@ -582,6 +582,17 @@ type grpcMarshaler struct {
 func (m *grpcMarshaler) MarshalWebTrailers(trailer http.Header) *Error {
 	raw := m.envelopeWriter.bufferPool.Get()
 	defer m.envelopeWriter.bufferPool.Put(raw)
+	for key, values := range trailer {
+		// Per the Go specification, keys inserted during iteration may be produced
+		// later in the iteration or may be skipped. For safety, avoid mutating the
+		// map if the key is already lower-cased.
+		lower := strings.ToLower(key)
+		if key == lower {
+			continue
+		}
+		delete(trailer, key)
+		trailer[lower] = values
+	}
 	if err := trailer.Write(raw); err != nil {
 		return errorf(CodeInternal, "format trailers: %w", err)
 	}
diff --git a/protocol_grpc_test.go b/protocol_grpc_test.go
index 726e9e24..8391a1f0 100644
--- a/protocol_grpc_test.go
+++ b/protocol_grpc_test.go
@@ -174,3 +174,23 @@ func TestGRPCPercentEncoding(t *testing.T) {
 	roundtrip(`foo%bar`)
 	roundtrip("fiancée")
 }
+
+func TestGRPCWebTrailerMarshalling(t *testing.T) {
+	t.Parallel()
+	responseWriter := httptest.NewRecorder()
+	marshaler := grpcMarshaler{
+		envelopeWriter: envelopeWriter{
+			writer:     responseWriter,
+			bufferPool: newBufferPool(),
+		},
+	}
+	trailer := http.Header{}
+	trailer.Add("grpc-status", "0")
+	trailer.Add("Grpc-Message", "Foo")
+	trailer.Add("User-Provided", "bar")
+	err := marshaler.MarshalWebTrailers(trailer)
+	assert.Nil(t, err)
+	responseWriter.Body.Next(5) // skip flags and message length
+	marshalled := responseWriter.Body.String()
+	assert.Equal(t, marshalled, "grpc-message: Foo\r\ngrpc-status: 0\r\nuser-provided: bar\r\n")
+}