diff --git a/R/firmly.R b/R/firmly.R
index c904afb..0df3208 100644
--- a/R/firmly.R
+++ b/R/firmly.R
@@ -70,12 +70,11 @@ warning_closure <- function(.fn, .warn) {
   force(.warn)
 
   function() {
-    call <- match.call()
-    .warn(call)
+    call <- base::match.call()
+    encl <- base::parent.env(base::environment())
+    encl$.warn(call)
 
-    parent <- parent.frame()
-    encl <- parent.env(environment())
-    eval(encl$.fn(call), parent)
+    base::eval.parent(encl$.fn(call))
   }
 }
 
@@ -108,33 +107,42 @@ validating_closure <- function(.chks, .sig, .fn, .warn, .error_class) {
   force(.warn)
   force(.error_class)
 
+  exprs <- purrr::transpose(.chks[c("expr", "env")])
   error <- function(message) {
     structure(
       list(message = message, call = NULL),
       class = c(.error_class, "error", "condition")
     )
   }
-  exprs <- purrr::transpose(.chks[c("expr", "env")])
+
+  # Local bindings to avoid (unlikely) clashes with formal arguments
+  deparse_collapse <- match.fun("deparse_collapse")
+  enumerate_many   <- match.fun("enumerate_many")
+  problems         <- match.fun("problems")
+  promises         <- match.fun("promises")
 
   function() {
-    call <- match.call()
-    .warn(call)
-
-    parent <- parent.frame()
-    encl <- parent.env(environment())
-    env <- promises(call, encl$.sig, parent)
-    verdict <- suppressWarnings(lapply(encl$exprs, function(.)
-      tryCatch(eval(.$expr, `parent.env<-`(env, .$env)), error = identity)
+    call <- base::match.call()
+    encl <- base::parent.env(base::environment())
+    encl$.warn(call)
+
+    parent <- base::parent.frame()
+    env <- encl$promises(call, encl$.sig, parent)
+    verdict <- base::suppressWarnings(base::lapply(encl$exprs, function(.)
+      base::tryCatch(base::eval(.$expr, base::`parent.env<-`(env, .$env)),
+                     error = base::identity)
     ))
-    pass <- vapply(verdict, is_true, logical(1))
+    pass <- base::vapply(verdict, base::isTRUE, base::logical(1))
 
-    if (all(pass)) {
-      eval(encl$.fn(call), parent)
+    if (base::all(pass)) {
+      base::eval(encl$.fn(call), parent)
     } else {
       fail <- !pass
-      msg_call  <- sprintf("%s\n", deparse_collapse(call))
-      msg_error <- enumerate_many(problems(encl$.chks[fail, ], verdict[fail]))
-      stop(encl$error(paste0(msg_call, msg_error)))
+      msg_call  <- base::sprintf("%s\n", encl$deparse_collapse(call))
+      msg_error <- encl$enumerate_many(
+        encl$problems(encl$.chks[fail, ], verdict[fail])
+      )
+      base::stop(encl$error(base::paste0(msg_call, msg_error)))
     }
   }
 }
diff --git a/tests/testthat/test-validation.R b/tests/testthat/test-validation.R
index a78b491..14bc43e 100644
--- a/tests/testthat/test-validation.R
+++ b/tests/testthat/test-validation.R
@@ -391,41 +391,51 @@ test_that("predicate is evaluated in its ambient formula environment", {
   }
 })
 
-test_that("names in checking procedure don't override function arguments", {
+test_that("formal arguments don't override names in validation procedure", {
   # Bindings in execution/enclosing environment of validating_closure()
-  nms <- c("call", "parent", "encl", "env", "verdict", "pass", "fail",
-           "msg_call", "msg_error", ".chks", ".sig", ".warn", "exprs")
+  nms    <- c("call", "parent", "encl", "env", "verdict", "pass", "fail",
+              "msg_call", "msg_error", ".chks", ".sig", "exprs")
+  nms_fn <- c(".fn", ".warn", "deparse_collapse", "enumerate_many", "error",
+              "problems", "promises")
 
   sum_args <- parse(text = paste(nms, collapse = "+"))
   f <- function() {
-    # Ensure that no arguments have type "logical"
+    # Ensure that no arguments have are logical (thus are coercible to numeric)
     args <- as.list(match.call()[-1])
     stopifnot(vapply(args, Negate(is.logical), logical(1)))
-
     eval(sum_args)
   }
 
-  # All arguments get an positive integer default value, except .fn
-  wrong_fn <- function(x) "Wrong function called"
-  def_args_nonfn <- seq_along(nms)
-  def_args <- c(alist(.fn = wrong_fn), stats::setNames(def_args_nonfn, nms))
+  # All function arguments get the function wrong_fn as default value
+  wrong_fn <- function(...) {
+    message("This function shouldn't have been called!")
+  }
+  def_args_fn <- stats::setNames(vector("list", length(nms_fn)), nms_fn)
+  def_args_fn[] <- list(quote(wrong_fn))
+
+  # All non-function arguments get an positive integer default value
+  def_args_nonfn <- stats::setNames(seq_along(nms), nms)
+  def_args <- c(def_args_fn, def_args_nonfn)
   formals(f) <- as.pairlist(def_args)
 
-  f_firm <- firmly(
-    f,
-    list(~.fn) ~ is.function,
-    lapply(nms, function(.) eval(parse(text = paste0("~", .)))) ~ is.numeric
-  )
+  make_checkfml <- function(.) eval(parse(text = paste0("~", .)))
+  f_firm <- firmly(f,
+                   lapply(nms_fn, make_checkfml) ~ is.function,
+                   lapply(nms, make_checkfml) ~ is.numeric)
   f_warn <- firmly(f, .warn_missing = names(def_args))
 
-  # Verify that f_firm(), f_warn() generate no errors and has correct value
+  # Verify that f_firm(), f_warn() generate no errors and return correct value
   expect_error(f_firm(), NA)
   expect_error(do.call("f_warn", def_args), NA)
   sum(def_args_nonfn) %>%
     expect_identical(f_firm()) %>%
     expect_identical(do.call("f_warn", def_args))
 
-  # Verify that f_firm(), f_warn() do not (accidentally) call .fn
-  expect_false(isTRUE(all.equal(f_firm(), wrong_fn())))
-  expect_false(isTRUE(all.equal(do.call("f_warn", def_args), wrong_fn())))
+  # Verify that f_firm(), f_warn() do not call its function arguments
+  msg <- capture_messages(wrong_fn())
+  expect_gt(length(msg), 0)        # msg is non-empty string
+  expect_message(wrong_fn(), msg)  # wrong_fn() produces msg when called
+  # f_firm(), f_warn() produce no message, so wrong_fn() wasn't called
+  expect_message(f_firm(), NA)
+  expect_message(do.call("f_warn", def_args), NA)
 })