diff --git a/api/http/error.go b/api/http/error.go
index e7ab0cd..eb46827 100644
--- a/api/http/error.go
+++ b/api/http/error.go
@@ -46,12 +46,16 @@ func (e *Error) HeaderToString(selected []string) string {
 	headerString := ""
 	if len(selected) == 0 {
 		for key := range e.Header {
-			headerString += fmt.Sprintf("%s: %s\r\n", key, e.Header[key])
+			headerString += fmt.Sprintf("%s: %s\r\n",
+				http.CanonicalHeaderKey(key),
+				e.Header.Get(key))
 		}
 	} else {
 		for _, candidate := range selected {
 			if e.Header.Get(candidate) != "" {
-				headerString += fmt.Sprintf("%s: %s\n", candidate, e.Header.Get(candidate))
+				headerString += fmt.Sprintf("%s: %s\n",
+					http.CanonicalHeaderKey(candidate),
+					e.Header.Get(candidate))
 			}
 		}
 	}
diff --git a/api/write_test.go b/api/write_test.go
index 4fda86b..a3c6540 100644
--- a/api/write_test.go
+++ b/api/write_test.go
@@ -312,3 +312,44 @@ func TestWriteApiErrorHeaders(t *testing.T) {
 	wg.Wait()
 	assert.Equal(t, calls, 3)
 }
+
+func TestWriteErrorHeaderToString(t *testing.T) {
+	header := ihttp.Header{
+		"Date":           []string{"2024-08-07T12:00:00.009"},
+		"Content-Length": []string{"12"},
+		"Content-Type":   []string{"application/json", "encoding UTF-8"},
+		"X-Test-Value1":  []string{"SaturnV"},
+		"X-Test-Value2":  []string{"Apollo11"},
+		"Retry-After":    []string{"2044"},
+		"Trace-Id":       []string{"123456789ABCDEF0"},
+	}
+
+	err := http.Error{
+		StatusCode: ihttp.StatusBadRequest,
+		Code:       "bad request",
+		Message:    "this is just a test",
+		Err:        nil,
+		RetryAfter: 2044,
+		Header:     header,
+	}
+
+	fullString := err.HeaderToString([]string{})
+
+	// write order is not guaranteed
+	assert.Contains(t, fullString, "Date: 2024-08-07T12:00:00.009")
+	assert.Contains(t, fullString, "Content-Length: 12")
+	assert.Contains(t, fullString, "Content-Type: application/json")
+	assert.Contains(t, fullString, "X-Test-Value1: SaturnV")
+	assert.Contains(t, fullString, "X-Test-Value2: Apollo11")
+	assert.Contains(t, fullString, "Retry-After: 2044")
+	assert.Contains(t, fullString, "Trace-Id: 123456789ABCDEF0")
+
+	filterString := err.HeaderToString([]string{"date", "trace-id", "x-test-value1", "x-test-value2"})
+
+	// write order will follow filter arguments
+	assert.Equal(t, filterString,
+		"Date: 2024-08-07T12:00:00.009\nTrace-Id: 123456789ABCDEF0\nX-Test-Value1: SaturnV\nX-Test-Value2: Apollo11\n",
+	)
+	assert.NotContains(t, filterString, "Content-Type: application/json")
+	assert.NotContains(t, filterString, "Retry-After: 2044")
+}