diff --git a/gzhttp/asserts_test.go b/gzhttp/asserts_test.go
index 987b848ea0..5bbfd80227 100644
--- a/gzhttp/asserts_test.go
+++ b/gzhttp/asserts_test.go
@@ -15,7 +15,7 @@ func assertEqual(t testing.TB, want, got interface{}) {
 func assertNotEqual(t testing.TB, want, got interface{}) {
 	t.Helper()
 	if reflect.DeepEqual(want, got) {
-		t.Fatalf("want %#v, got %#v", want, got)
+		t.Fatalf("did not want %#v, got %#v", want, got)
 	}
 }
 
diff --git a/gzhttp/gzip.go b/gzhttp/gzip.go
index dcca6288c8..759845a9e5 100644
--- a/gzhttp/gzip.go
+++ b/gzhttp/gzip.go
@@ -97,6 +97,12 @@ func (w *GzipResponseWriter) Write(b []byte) (int, error) {
 			// If a Content-Type wasn't specified, infer it from the current buffer.
 			if ct == "" {
 				ct = http.DetectContentType(w.buf)
+			}
+
+			// Handles the intended case of setting a nil Content-Type (as for http/server or http/fs)
+			// Set the header only if the key does not exist
+			_, haveType := w.Header()["Content-Type"]
+			if !haveType {
 				w.Header().Set(contentType, ct)
 			}
 			// If the Content-Type is acceptable to GZIP, initialize the GZIP writer.
diff --git a/gzhttp/gzip_test.go b/gzhttp/gzip_test.go
index ca5ab9f909..e33e8fa887 100644
--- a/gzhttp/gzip_test.go
+++ b/gzhttp/gzip_test.go
@@ -649,3 +649,18 @@ func newTestHandler(body string) http.Handler {
 		}
 	}))
 }
+
+func TestGzipHandlerNilContentType(t *testing.T) {
+	// This just exists to provide something for GzipHandler to wrap.
+	handler := newTestHandler(testBody)
+
+	// content-type header not set when provided nil
+
+	req, _ := http.NewRequest("GET", "/whatever", nil)
+	req.Header.Set("Accept-Encoding", "gzip")
+	res := httptest.NewRecorder()
+	res.Header()["Content-Type"] = nil
+	handler.ServeHTTP(res, req)
+
+	assertEqual(t, "", res.Header().Get("Content-Type"))
+}