diff --git a/NEWS.md b/NEWS.md
index 6f7164f421..284f0fd9d5 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -50,6 +50,8 @@ in shiny apps. For more info, see the documentation (`?updateQueryString` and `?
 
 * Fixed [#1605](https://github.com/rstudio/shiny/issues/1605): sliders did not move when clicked on the bar area. ([#1610](https://github.com/rstudio/shiny/pull/1610))
 
+* Fixed [#1621](https://github.com/rstudio/shiny/issues/1621): if a `reactiveTimer`'s session was closed before the first time that the `reactiveTimer` fired, then the `reactiveTimer` would not get cleared and would keep firing indefinitely. ([#1623](https://github.com/rstudio/shiny/pull/1623))
+
 ### Library updates
 
 * Closed [#1500](https://github.com/rstudio/shiny/issues/1500): Updated ion.rangeSlider to 2.1.6. ([#1540](https://github.com/rstudio/shiny/pull/1540))
diff --git a/R/reactives.R b/R/reactives.R
index 51a7100a13..8ccea85dd2 100644
--- a/R/reactives.R
+++ b/R/reactives.R
@@ -1151,6 +1151,10 @@ setAutoflush <- local({
 #' }
 #' @export
 reactiveTimer <- function(intervalMs=1000, session = getDefaultReactiveDomain()) {
+  # Need to make sure that session is resolved at creation, not when the
+  # callback below is fired (see #1614).
+  force(session)
+
   dependents <- Map$new()
   timerCallbacks$schedule(intervalMs, function() {
     # Quit if the session is closed