diff --git a/DESCRIPTION b/DESCRIPTION
index 91396f21..bfc2d974 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -19,7 +19,7 @@ Depends:
 Imports: 
     glue,
     magrittr,
-    stringi (>= 0.4.1)
+    stringi (>= 1.1.6)
 Suggests: 
     covr,
     htmltools,
diff --git a/NEWS.md b/NEWS.md
index 547e4d5f..432bb537 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -13,6 +13,10 @@
   `str_replace()` and `str_replace_all()` to remove patterns from strings.
   (@Shians, #178)
 
+* `str_sub()` gains `omit_na` argument for ignoring `NA`. Accordingly,
+  `str_replace()` now ignores `NA`s and keeps the original strings.
+  (@yutannihilation, #164)
+
 ## Bug fixes and minor improvements
 
 * `str_trunc()` now preserves NAs (@ClaytonJY, #162)
diff --git a/R/replace.r b/R/replace.r
index afe4693d..6915c325 100644
--- a/R/replace.r
+++ b/R/replace.r
@@ -159,7 +159,7 @@ str_replace_na <- function(string, replacement = "NA") {
 
 str_transform <- function(string, pattern, replacement) {
   loc <- str_locate(string, pattern)
-  str_sub(string, loc) <- replacement(str_sub(string, loc))
+  str_sub(string, loc, omit_na = TRUE) <- replacement(str_sub(string, loc))
   string
 }
 str_transform_all <- function(string, pattern, replacement) {
diff --git a/R/sub.r b/R/sub.r
index 616bfe01..d318d35e 100644
--- a/R/sub.r
+++ b/R/sub.r
@@ -15,6 +15,8 @@
 #'   matrix to `start`.
 #'
 #'   Negative values count backwards from the last character.
+#' @param omit_na Single logical value. If `TRUE`, missing values in any of the
+#'   arguments provided will result in an unchanged input.
 #' @param value replacement string
 #' @return A character vector of substring from `start` to `end`
 #'   (inclusive). Will be length of longest input argument.
@@ -50,6 +52,15 @@
 #' str_sub(x, -1, -1) <- "K"; x
 #' str_sub(x, -2, -2) <- "GHIJ"; x
 #' str_sub(x, 2, -2) <- ""; x
+#'
+#' # If you want to keep the original if some argument is NA,
+#' # use omit_na = TRUE
+#' x1 <- x2 <- x3 <- x4 <- "AAA"
+#' str_sub(x1, 1, NA) <- "B"
+#' str_sub(x2, 1, 2) <- NA
+#' str_sub(x3, 1, NA, omit_na = TRUE) <- "B"
+#' str_sub(x4, 1, 2, omit_na = TRUE) <- NA
+#' x1; x2; x3; x4
 str_sub <- function(string, start = 1L, end = -1L) {
   if (is.matrix(start)) {
     stri_sub(string, from = start)
@@ -61,11 +72,11 @@ str_sub <- function(string, start = 1L, end = -1L) {
 
 #' @export
 #' @rdname str_sub
-"str_sub<-" <- function(string, start = 1L, end = -1L, value) {
+"str_sub<-" <- function(string, start = 1L, end = -1L, omit_na = FALSE,  value) {
   if (is.matrix(start)) {
-    stri_sub(string, from = start) <- value
+    stri_sub(string, from = start, omit_na = omit_na) <- value
   } else {
-    stri_sub(string, from = start, to = end) <- value
+    stri_sub(string, from = start, to = end, omit_na = omit_na) <- value
   }
   string
 }
diff --git a/man/str_sub.Rd b/man/str_sub.Rd
index 5b53a719..1a969aa1 100644
--- a/man/str_sub.Rd
+++ b/man/str_sub.Rd
@@ -7,7 +7,7 @@
 \usage{
 str_sub(string, start = 1L, end = -1L)
 
-str_sub(string, start = 1L, end = -1L) <- value
+str_sub(string, start = 1L, end = -1L, omit_na = FALSE) <- value
 }
 \arguments{
 \item{string}{input character vector.}
@@ -19,6 +19,9 @@ matrix to \code{start}.
 
 Negative values count backwards from the last character.}
 
+\item{omit_na}{Single logical value. If \code{TRUE}, missing values in any of the
+arguments provided will result in an unchanged input.}
+
 \item{value}{replacement string}
 }
 \value{
@@ -65,6 +68,15 @@ str_sub(x, 1, 1) <- "A"; x
 str_sub(x, -1, -1) <- "K"; x
 str_sub(x, -2, -2) <- "GHIJ"; x
 str_sub(x, 2, -2) <- ""; x
+
+# If you want to keep the original if some argument is NA,
+# use omit_na = TRUE
+x1 <- x2 <- x3 <- x4 <- "AAA"
+str_sub(x1, 1, NA) <- "B"
+str_sub(x2, 1, 2) <- NA
+str_sub(x3, 1, NA, omit_na = TRUE) <- "B"
+str_sub(x4, 1, 2, omit_na = TRUE) <- NA
+x1; x2; x3; x4
 }
 \seealso{
 The underlying implementation in \code{\link[stringi:stri_sub]{stringi::stri_sub()}}
diff --git a/tests/testthat/test-replace.r b/tests/testthat/test-replace.r
index ffee0f47..1cef7f95 100644
--- a/tests/testthat/test-replace.r
+++ b/tests/testthat/test-replace.r
@@ -69,6 +69,10 @@ test_that("replacement can be different length", {
   expect_equal(str_replace_all("abc", "a|c", double), "aabcc")
 })
 
+test_that("replacement with NA works", {
+  expect_equal(str_replace("abc", "z", toupper), "abc")
+})
+
 # fix_replacement ---------------------------------------------------------
 
 test_that("$ are escaped", {
diff --git a/tests/testthat/test-sub.r b/tests/testthat/test-sub.r
index 92c87338..110d7d74 100644
--- a/tests/testthat/test-sub.r
+++ b/tests/testthat/test-sub.r
@@ -69,6 +69,15 @@ test_that("replacement works", {
 
   str_sub(x, 2, -2) <- ""
   expect_equal(x, "AH")
+})
 
+test_that("replacement with NA works", {
+  x <- "BBCDEF"
+  str_sub(x, NA) <- "A"
+  expect_equal(x, NA_character_)
 
+  x <- "BBCDEF"
+  str_sub(x, NA, omit_na = TRUE) <- "A"
+  str_sub(x, 1, 1, omit_na = TRUE) <- NA
+  expect_equal(x, "BBCDEF")
 })