diff --git a/api/prometheus/v1/api.go b/api/prometheus/v1/api.go
index c41c0cf..192ba49 100644
--- a/api/prometheus/v1/api.go
+++ b/api/prometheus/v1/api.go
@@ -455,6 +455,11 @@ type apiResponse struct {
 	Error     string          `json:"error"`
 }
 
+func apiError(code int) bool {
+	// These are the codes that Prometheus sends when it returns an error.
+	return code == statusAPIError || code == http.StatusBadRequest
+}
+
 func (c apiClient) Do(ctx context.Context, req *http.Request) (*http.Response, []byte, error) {
 	resp, body, err := c.Client.Do(ctx, req)
 	if err != nil {
@@ -463,7 +468,7 @@ func (c apiClient) Do(ctx context.Context, req *http.Request) (*http.Response, [
 
 	code := resp.StatusCode
 
-	if code/100 != 2 && code != statusAPIError {
+	if code/100 != 2 && !apiError(code) {
 		return resp, body, &Error{
 			Type: ErrBadResponse,
 			Msg:  fmt.Sprintf("bad response code %d", resp.StatusCode),
@@ -479,14 +484,14 @@ func (c apiClient) Do(ctx context.Context, req *http.Request) (*http.Response, [
 		}
 	}
 
-	if (code == statusAPIError) != (result.Status == "error") {
+	if apiError(code) != (result.Status == "error") {
 		err = &Error{
 			Type: ErrBadResponse,
 			Msg:  "inconsistent body for response code",
 		}
 	}
 
-	if code == statusAPIError && result.Status == "error" {
+	if apiError(code) && result.Status == "error" {
 		err = &Error{
 			Type: result.ErrorType,
 			Msg:  result.Error,
diff --git a/api/prometheus/v1/api_test.go b/api/prometheus/v1/api_test.go
index 2fe6044..707d29a 100644
--- a/api/prometheus/v1/api_test.go
+++ b/api/prometheus/v1/api_test.go
@@ -606,7 +606,19 @@ func TestAPIClientDo(t *testing.T) {
 			response: "bad json",
 			err: &Error{
 				Type: ErrBadResponse,
-				Msg:  "bad response code 400",
+				Msg:  "bad response code 500",
+			},
+			code: http.StatusInternalServerError,
+		},
+		{
+			response: &apiResponse{
+				Status:    "error",
+				ErrorType: ErrBadData,
+				Error:     "end timestamp must not be before start time",
+			},
+			err: &Error{
+				Type: ErrBadData,
+				Msg:  "end timestamp must not be before start time",
 			},
 			code: http.StatusBadRequest,
 		},