diff --git a/promql/engine.go b/promql/engine.go
index 1cdac0828..7d27d586d 100644
--- a/promql/engine.go
+++ b/promql/engine.go
@@ -217,6 +217,9 @@ type (
 	ErrQueryTimeout string
 	// ErrQueryCanceled is returned if a query was canceled during processing.
 	ErrQueryCanceled string
+	// ErrStorage is returned if an error was encountered in the storage layer
+	// during query handling.
+	ErrStorage error
 )
 
 func (e ErrQueryTimeout) Error() string  { return fmt.Sprintf("query timed out in %s", string(e)) }
diff --git a/web/api/v1/api.go b/web/api/v1/api.go
index 4f3e25f78..7cc588065 100644
--- a/web/api/v1/api.go
+++ b/web/api/v1/api.go
@@ -50,6 +50,7 @@ const (
 	errorCanceled           = "canceled"
 	errorExec               = "execution"
 	errorBadData            = "bad_data"
+	errorInternal           = "internal"
 )
 
 var corsHeaders = map[string]string{
@@ -194,6 +195,8 @@ func (api *API) query(r *http.Request) (interface{}, *apiError) {
 			return nil, &apiError{errorCanceled, res.Err}
 		case promql.ErrQueryTimeout:
 			return nil, &apiError{errorTimeout, res.Err}
+		case promql.ErrStorage:
+			return nil, &apiError{errorInternal, res.Err}
 		}
 		return nil, &apiError{errorExec, res.Err}
 	}
@@ -459,6 +462,8 @@ func respondError(w http.ResponseWriter, apiErr *apiError, data interface{}) {
 		code = 422
 	case errorCanceled, errorTimeout:
 		code = http.StatusServiceUnavailable
+	case errorInternal:
+		code = http.StatusInternalServerError
 	default:
 		code = http.StatusInternalServerError
 	}