diff --git a/R/R/prophet.R b/R/R/prophet.R
index bbd4a49..4f33913 100644
--- a/R/R/prophet.R
+++ b/R/R/prophet.R
@@ -473,6 +473,9 @@ logistic_growth_init <- function(df) {
 #'
 #' @export
 fit.prophet <- function(m, df, ...) {
+  if (!is.null(m$history)) {
+    stop("Prophet object can only be fit once. Instantiate a new object.")
+  }
   history <- df %>%
     dplyr::filter(!is.na(y))
   m$history.dates <- sort(zoo::as.Date(df$ds))
diff --git a/python/fbprophet/forecaster.py b/python/fbprophet/forecaster.py
index 65529be..919b587 100644
--- a/python/fbprophet/forecaster.py
+++ b/python/fbprophet/forecaster.py
@@ -479,6 +479,9 @@ class Prophet(object):
         -------
         The fitted Prophet object.
         """
+        if self.history is not None:
+            raise Exception('Prophet object can only be fit once. '
+                            'Instantiate a new object.')
         history = df[df['y'].notnull()].copy()
         self.history_dates = pd.to_datetime(df['ds']).sort_values()