diff --git a/R/expect-that.R b/R/expect-that.R
index 730f3b2fb..2c09edb5f 100644
--- a/R/expect-that.R
+++ b/R/expect-that.R
@@ -8,8 +8,9 @@
 #' Learn more about creating your own expectations in
 #' `vignette("custom-expectation")`.
 #'
-#' @param message Failure message to send to the user. It's best practice to
-#'   describe both what is expected and what was actually received.
+#' @param message A character vector describing the failure. The
+#'   first element should describe the expected value, and the second (and
+#'   optionally subsequence) elements should describe what was actually seen.
 #' @param info Character vector continuing additional information. Included
 #'   for backward compatibility only and new expectations should not use it.
 #' @param srcref Location of the failure. Should only needed to be explicitly
@@ -41,6 +42,9 @@ fail <- function(
   trace_env = caller_env(),
   trace = NULL
 ) {
+  check_character(message)
+  check_character(info, allow_null = TRUE)
+
   trace <- trace %||% capture_trace(trace_env)
   message <- paste(c(message, info), collapse = "\n")
   expectation("failure", message, srcref = srcref, trace = trace)
diff --git a/R/expectation.R b/R/expectation.R
index 55f00845a..a155a3430 100644
--- a/R/expectation.R
+++ b/R/expectation.R
@@ -5,7 +5,9 @@
 #' `vignette("custom-expectation")` for details.
 #'
 #' @param ok `TRUE` or `FALSE` indicating if the expectation was successful.
-#' @param failure_message Message to show if the expectation failed.
+#' @param failure_message A character vector describing the failure. The
+#'   first element should describe the expected value, and the second (and
+#'   optionally subsequence) elements should describe what was actually seen.
 #' @inheritParams fail
 #' @return An expectation object from either `succeed()` or `fail()`.
 #'   with a `continue_test` restart.
@@ -20,6 +22,9 @@ expect <- function(
   trace = NULL,
   trace_env = caller_env()
 ) {
+  check_bool(ok)
+  check_character(failure_message)
+
   if (!ok) {
     return(fail(
       failure_message,
diff --git a/man/expect.Rd b/man/expect.Rd
index b1da702e3..2b0c29fab 100644
--- a/man/expect.Rd
+++ b/man/expect.Rd
@@ -16,7 +16,9 @@ expect(
 \arguments{
 \item{ok}{\code{TRUE} or \code{FALSE} indicating if the expectation was successful.}
 
-\item{failure_message}{Message to show if the expectation failed.}
+\item{failure_message}{A character vector describing the failure. The
+first element should describe the expected value, and the second (and
+optionally subsequence) elements should describe what was actually seen.}
 
 \item{info}{Character vector continuing additional information. Included
 for backward compatibility only and new expectations should not use it.}
diff --git a/man/fail.Rd b/man/fail.Rd
index 09d9d4350..04d4cc446 100644
--- a/man/fail.Rd
+++ b/man/fail.Rd
@@ -16,8 +16,9 @@ fail(
 pass(value)
 }
 \arguments{
-\item{message}{Failure message to send to the user. It's best practice to
-describe both what is expected and what was actually received.}
+\item{message}{A character vector describing the failure. The
+first element should describe the expected value, and the second (and
+optionally subsequence) elements should describe what was actually seen.}
 
 \item{info}{Character vector continuing additional information. Included
 for backward compatibility only and new expectations should not use it.}
diff --git a/man/succeed.Rd b/man/succeed.Rd
index c05398609..772e19db4 100644
--- a/man/succeed.Rd
+++ b/man/succeed.Rd
@@ -7,8 +7,9 @@
 succeed(message = "Success has been forced", info = NULL)
 }
 \arguments{
-\item{message}{Failure message to send to the user. It's best practice to
-describe both what is expected and what was actually received.}
+\item{message}{A character vector describing the failure. The
+first element should describe the expected value, and the second (and
+optionally subsequence) elements should describe what was actually seen.}
 
 \item{info}{Character vector continuing additional information. Included
 for backward compatibility only and new expectations should not use it.}
diff --git a/tests/testthat/_snaps/expectation.md b/tests/testthat/_snaps/expectation.md
new file mode 100644
index 000000000..977def5f4
--- /dev/null
+++ b/tests/testthat/_snaps/expectation.md
@@ -0,0 +1,13 @@
+# validates key inputs
+
+    Code
+      expect(1)
+    Condition
+      Error in `expect()`:
+      ! `ok` must be `TRUE` or `FALSE`, not the number 1.
+    Code
+      expect(TRUE, 1)
+    Condition
+      Error in `expect()`:
+      ! `failure_message` must be a character vector, not the number 1.
+
diff --git a/tests/testthat/test-expectation.R b/tests/testthat/test-expectation.R
index 3e63e2ca2..39d02f07f 100644
--- a/tests/testthat/test-expectation.R
+++ b/tests/testthat/test-expectation.R
@@ -7,6 +7,13 @@ test_that("info only evaluated on failure", {
   expect_no_error(expect(TRUE, "fail", info = stop("!")))
 })
 
+test_that("validates key inputs", {
+  expect_snapshot(error = TRUE, {
+    expect(1)
+    expect(TRUE, 1)
+  })
+})
+
 test_that("can subclass expectation", {
   exp <- new_expectation(
     "failure",