diff --git a/R/slide.R b/R/slide.R
index 5a7fbd6a..b971b9d5 100644
--- a/R/slide.R
+++ b/R/slide.R
@@ -576,7 +576,7 @@ get_before_after_from_window <- function(window_size, align, time_type) {
 #'   ungroup()
 epi_slide_opt <- function(
     .x, .col_names, .f, ...,
-    .window_size = 1, .align = c("right", "center", "left"),
+    .window_size = NULL, .align = c("right", "center", "left"),
     .ref_time_values = NULL, .all_rows = FALSE) {
   assert_class(.x, "epi_df")
 
@@ -902,7 +902,7 @@ epi_slide_opt <- function(
 #'   ungroup()
 epi_slide_mean <- function(
     .x, .col_names, ...,
-    .window_size = 1, .align = c("right", "center", "left"),
+    .window_size = NULL, .align = c("right", "center", "left"),
     .ref_time_values = NULL, .all_rows = FALSE) {
   # Deprecated argument handling
   provided_args <- rlang::call_args_names(rlang::call_match())
@@ -979,7 +979,7 @@ epi_slide_mean <- function(
 #'   ungroup()
 epi_slide_sum <- function(
     .x, .col_names, ...,
-    .window_size = 1, .align = c("right", "center", "left"),
+    .window_size = NULL, .align = c("right", "center", "left"),
     .ref_time_values = NULL, .all_rows = FALSE) {
   # Deprecated argument handling
   provided_args <- rlang::call_args_names(rlang::call_match())
diff --git a/R/utils.R b/R/utils.R
index 88c7eaa0..32dcb492 100644
--- a/R/utils.R
+++ b/R/utils.R
@@ -982,14 +982,31 @@ guess_period.POSIXt <- function(time_values, time_values_arg = rlang::caller_arg
   as.numeric(NextMethod(), units = "secs")
 }
 
-validate_slide_window_arg <- function(arg, time_type, lower = 1, allow_inf = TRUE, arg_name = rlang::caller_arg(arg)) {
-  if (!checkmate::test_scalar(arg) || arg < lower) {
-    cli_abort(
-      "Slide function expected `{arg_name}` to be a non-null, scalar integer >= {lower}.",
-      class = "epiprocess__validate_slide_window_arg"
+#' Is `x` an "int" with a sensible class? TRUE/FALSE
+#'
+#' Like [`checkmate::test_int`] but disallowing some non-sensible classes that
+#' `test_int` accepts, such as `difftime`s. We rely on [`is.numeric`] to
+#' determine class appropriateness; note that `is.numeric` is NOT simply
+#' checking for the class to be "numeric" (or else we'd fail on integer class).
+#'
+#' @param x object
+#' @return Boolean
+#'
+#' @importFrom checkmate test_int
+#' @keywords internal
+test_sensible_int <- function(x, na.ok = FALSE, lower = -Inf, upper = Inf,
+                              tol = sqrt(.Machine$double.eps), null.ok = FALSE) {
+  if (null.ok && is.null(x)) {
+    TRUE
+  } else {
+    is.numeric(x) && test_int(x,
+      na.ok = FALSE, lower = -Inf, upper = Inf,
+      tol = sqrt(.Machine$double.eps)
     )
   }
+}
 
+validate_slide_window_arg <- function(arg, time_type, lower = 1, allow_inf = TRUE, arg_name = rlang::caller_arg(arg)) {
   if (time_type == "custom") {
     cli_abort(
       "Unsure how to interpret slide units with a custom time type. Consider converting your time
@@ -999,31 +1016,42 @@ validate_slide_window_arg <- function(arg, time_type, lower = 1, allow_inf = TRU
   }
 
   msg <- ""
-  if (!identical(arg, Inf)) {
-    if (time_type == "day") {
-      if (!test_int(arg, lower = 0L) || inherits(arg, "difftime") && units(arg) != "days") {
-        msg <- glue::glue_collapse(c("difftime with units in days", "non-negative integer", "Inf"), " or ")
-      }
-    } else if (time_type == "week") {
-      if (!(inherits(arg, "difftime") && units(arg) == "weeks")) {
-        msg <- glue::glue_collapse(c("difftime with units in weeks", "Inf"), " or ")
-      }
-    } else if (time_type == "yearmonth") {
-      if (!test_int(arg, lower = 0L) || inherits(arg, "difftime")) {
-        msg <- glue::glue_collapse(c("non-negative integer", "Inf"), " or ")
-      }
-    } else if (time_type == "integer") {
-      if (!test_int(arg, lower = 0L) || inherits(arg, "difftime")) {
-        msg <- glue::glue_collapse(c("non-negative integer", "Inf"), " or ")
-      }
-    } else {
-      msg <- glue::glue_collapse(c("difftime", "non-negative integer", "Inf"), " or ")
-    }
+  inf_if_okay <- if (allow_inf) {
+    "Inf"
   } else {
-    if (!allow_inf) {
-      msg <- glue::glue_collapse(c("a difftime", "a non-negative integer"), " or ")
+    character(0L)
+  }
+
+  if (time_type == "day") {
+    if (!(test_sensible_int(arg, lower = 0L) ||
+      inherits(arg, "difftime") && length(arg) == 1L && units(arg) == "days" ||
+      allow_inf && identical(arg, Inf)
+    )) {
+      msg <- glue::glue_collapse(c("length-1 difftime with units in days", "non-negative integer", inf_if_okay), " or ")
     }
+  } else if (time_type == "week") {
+    if (!(inherits(arg, "difftime") && length(arg) == 1L && units(arg) == "weeks" ||
+      allow_inf && identical(arg, Inf)
+    )) {
+      msg <- glue::glue_collapse(c("length-1 difftime with units in weeks", inf_if_okay), " or ")
+    }
+  } else if (time_type == "yearmonth") {
+    if (!(test_sensible_int(arg, lower = 0L) ||
+      allow_inf && identical(arg, Inf)
+    )) {
+      msg <- glue::glue_collapse(c("non-negative integer", inf_if_okay), " or ")
+    }
+  } else if (time_type == "integer") {
+    if (!(test_sensible_int(arg, lower = 0L) ||
+      allow_inf && identical(arg, Inf)
+    )) {
+      msg <- glue::glue_collapse(c("non-negative integer", inf_if_okay), " or ")
+    }
+  } else {
+    cli_abort('`epiprocess` internal error: unrecognized time_type: "{time_type}"',
+              class = "epiprocess__unrecognized_time_type")
   }
+
   if (msg != "") {
     cli_abort(
       "Slide function expected `{arg_name}` to be a {msg}.",
diff --git a/man-roxygen/basic-slide-params.R b/man-roxygen/basic-slide-params.R
index 8a63a817..dfa2512f 100644
--- a/man-roxygen/basic-slide-params.R
+++ b/man-roxygen/basic-slide-params.R
@@ -10,7 +10,8 @@
 #'     with units "days"
 #'   - if time_type is Date and the cadence is weekly, then `.window_size` must
 #'     be a difftime with units "weeks"
-#'   - if time_type is an integer, then `.window_size` must be an integer
+#'   - if time_type is an yearmonth or integer, then `.window_size` must be an
+#'     integer
 #'
 #' @param .align The alignment of the sliding window. If `right` (default), then
 #' the window has its end at the reference time; if `center`, then the window is
diff --git a/man/epi_slide.Rd b/man/epi_slide.Rd
index 74929eb1..8029e2a4 100644
--- a/man/epi_slide.Rd
+++ b/man/epi_slide.Rd
@@ -57,7 +57,8 @@ an integer (which will be interpreted in units of days) or a difftime
 with units "days"
 \item if time_type is Date and the cadence is weekly, then \code{.window_size} must
 be a difftime with units "weeks"
-\item if time_type is an integer, then \code{.window_size} must be an integer
+\item if time_type is an yearmonth or integer, then \code{.window_size} must be an
+integer
 }}
 
 \item{.align}{The alignment of the sliding window. If \code{right} (default), then
diff --git a/man/epi_slide_mean.Rd b/man/epi_slide_mean.Rd
index 09faefb6..75b83b10 100644
--- a/man/epi_slide_mean.Rd
+++ b/man/epi_slide_mean.Rd
@@ -8,7 +8,7 @@ epi_slide_mean(
   .x,
   .col_names,
   ...,
-  .window_size = 1,
+  .window_size = NULL,
   .align = c("right", "center", "left"),
   .ref_time_values = NULL,
   .all_rows = FALSE
@@ -44,7 +44,8 @@ an integer (which will be interpreted in units of days) or a difftime
 with units "days"
 \item if time_type is Date and the cadence is weekly, then \code{.window_size} must
 be a difftime with units "weeks"
-\item if time_type is an integer, then \code{.window_size} must be an integer
+\item if time_type is an yearmonth or integer, then \code{.window_size} must be an
+integer
 }}
 
 \item{.align}{The alignment of the sliding window. If \code{right} (default), then
diff --git a/man/epi_slide_opt.Rd b/man/epi_slide_opt.Rd
index dcaab3f8..24b813f0 100644
--- a/man/epi_slide_opt.Rd
+++ b/man/epi_slide_opt.Rd
@@ -10,7 +10,7 @@ epi_slide_opt(
   .col_names,
   .f,
   ...,
-  .window_size = 1,
+  .window_size = NULL,
   .align = c("right", "center", "left"),
   .ref_time_values = NULL,
   .all_rows = FALSE
@@ -59,7 +59,8 @@ an integer (which will be interpreted in units of days) or a difftime
 with units "days"
 \item if time_type is Date and the cadence is weekly, then \code{.window_size} must
 be a difftime with units "weeks"
-\item if time_type is an integer, then \code{.window_size} must be an integer
+\item if time_type is an yearmonth or integer, then \code{.window_size} must be an
+integer
 }}
 
 \item{.align}{The alignment of the sliding window. If \code{right} (default), then
diff --git a/man/epi_slide_sum.Rd b/man/epi_slide_sum.Rd
index 0c83c432..2cf05cca 100644
--- a/man/epi_slide_sum.Rd
+++ b/man/epi_slide_sum.Rd
@@ -8,7 +8,7 @@ epi_slide_sum(
   .x,
   .col_names,
   ...,
-  .window_size = 1,
+  .window_size = NULL,
   .align = c("right", "center", "left"),
   .ref_time_values = NULL,
   .all_rows = FALSE
@@ -44,7 +44,8 @@ an integer (which will be interpreted in units of days) or a difftime
 with units "days"
 \item if time_type is Date and the cadence is weekly, then \code{.window_size} must
 be a difftime with units "weeks"
-\item if time_type is an integer, then \code{.window_size} must be an integer
+\item if time_type is an yearmonth or integer, then \code{.window_size} must be an
+integer
 }}
 
 \item{.align}{The alignment of the sliding window. If \code{right} (default), then
diff --git a/man/test_sensible_int.Rd b/man/test_sensible_int.Rd
new file mode 100644
index 00000000..eda7aacb
--- /dev/null
+++ b/man/test_sensible_int.Rd
@@ -0,0 +1,28 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/utils.R
+\name{test_sensible_int}
+\alias{test_sensible_int}
+\title{Is \code{x} an "int" with a sensible class? TRUE/FALSE}
+\usage{
+test_sensible_int(
+  x,
+  na.ok = FALSE,
+  lower = -Inf,
+  upper = Inf,
+  tol = sqrt(.Machine$double.eps),
+  null.ok = FALSE
+)
+}
+\arguments{
+\item{x}{object}
+}
+\value{
+Boolean
+}
+\description{
+Like \code{\link[checkmate:checkInt]{checkmate::test_int}} but disallowing some non-sensible classes that
+\code{test_int} accepts, such as \code{difftime}s. We rely on \code{\link{is.numeric}} to
+determine class appropriateness; note that \code{is.numeric} is NOT simply
+checking for the class to be "numeric" (or else we'd fail on integer class).
+}
+\keyword{internal}
diff --git a/tests/testthat/test-utils.R b/tests/testthat/test-utils.R
index 5767751d..a248a041 100644
--- a/tests/testthat/test-utils.R
+++ b/tests/testthat/test-utils.R
@@ -306,16 +306,22 @@ test_that("validate_slide_window_arg works", {
   }
   expect_no_error(validate_slide_window_arg(as.difftime(1, units = "days"), "day"))
   expect_no_error(validate_slide_window_arg(1, "day"))
-  expect_error(validate_slide_window_arg(as.difftime(1, units = "weeks"), "day"))
-  expect_error(validate_slide_window_arg(as.difftime(1, units = "secs"), "day"))
+  expect_error(validate_slide_window_arg(as.difftime(1, units = "weeks"), "day"),
+               class = "epiprocess__validate_slide_window_arg")
+  expect_error(validate_slide_window_arg(as.difftime(1, units = "secs"), "day"),
+               class = "epiprocess__validate_slide_window_arg")
 
   expect_no_error(validate_slide_window_arg(as.difftime(1, units = "weeks"), "week"))
-  expect_error(validate_slide_window_arg(1, "week"))
+  expect_error(validate_slide_window_arg(1, "week"),
+               class = "epiprocess__validate_slide_window_arg")
 
   expect_no_error(validate_slide_window_arg(1, "integer"))
-  expect_error(validate_slide_window_arg(as.difftime(1, units = "days"), "integer"))
-  expect_error(validate_slide_window_arg(as.difftime(1, units = "weeks"), "integer"))
+  expect_error(validate_slide_window_arg(as.difftime(1, units = "days"), "integer"),
+               class = "epiprocess__validate_slide_window_arg")
+  expect_error(validate_slide_window_arg(as.difftime(1, units = "weeks"), "integer"),
+               class = "epiprocess__validate_slide_window_arg")
 
   expect_no_error(validate_slide_window_arg(1, "yearmonth"))
-  expect_error(validate_slide_window_arg(as.difftime(1, units = "weeks"), "yearmonth"))
+  expect_error(validate_slide_window_arg(as.difftime(1, units = "weeks"), "yearmonth"),
+               class = "epiprocess__validate_slide_window_arg")
 })