diff --git a/R/fixed_design.R b/R/fixed_design.R
index 41aed81c..e3f72e95 100644
--- a/R/fixed_design.R
+++ b/R/fixed_design.R
@@ -547,7 +547,7 @@ fixed_design <- function(method = c("ahr", "fh", "mb", "lf", "rd", "maxcombo", "
           alpha = alpha, beta = 1 - power, ratio = ratio,
           enroll_rate = enroll_rate, fail_rate = fail_rate,
           analysis_time = study_duration,
-          test = "rmst difference",
+          test = "rmst_difference",
           tau = ifelse(has_tau, args$tau, study_duration)
         )
       } else {
@@ -555,7 +555,7 @@ fixed_design <- function(method = c("ahr", "fh", "mb", "lf", "rd", "maxcombo", "
           alpha = alpha, ratio = ratio,
           enroll_rate = enroll_rate, fail_rate = fail_rate,
           analysis_time = study_duration,
-          test = "rmst difference",
+          test = "rmst_difference",
           tau = ifelse(has_tau, args$tau, study_duration)
         )
       }
@@ -583,7 +583,7 @@ fixed_design <- function(method = c("ahr", "fh", "mb", "lf", "rd", "maxcombo", "
           alpha = alpha, beta = 1 - power, ratio = ratio,
           enroll_rate = enroll_rate, fail_rate = fail_rate,
           analysis_time = study_duration,
-          test = "survival difference",
+          test = "survival_difference",
           tau = ifelse(has_tau, args$tau, study_duration)
         )
       } else {
@@ -591,7 +591,7 @@ fixed_design <- function(method = c("ahr", "fh", "mb", "lf", "rd", "maxcombo", "
           alpha = alpha, ratio = ratio,
           enroll_rate = enroll_rate, fail_rate = fail_rate,
           analysis_time = study_duration,
-          test = "survival difference",
+          test = "survival_difference",
           tau = ifelse(has_tau, args$tau, study_duration)
         )
       }
@@ -615,6 +615,7 @@ fixed_design <- function(method = c("ahr", "fh", "mb", "lf", "rd", "maxcombo", "
     }
   )
 
+
   class(y) <- c("fixed_design", class(y))
   return(y)
 }
diff --git a/R/rmst.R b/R/rmst.R
index 81cd2b53..57f0a287 100644
--- a/R/rmst.R
+++ b/R/rmst.R
@@ -26,8 +26,8 @@
 #' @param beta Power (`NULL` to compute power or strictly between
 #'   0 and `1 - alpha` otherwise).
 #' @param test A string specifies the type of statistical test.
-#'   Default is `"survival difference"` (a Kaplan-Meier based test).
-#'   One can also set it as `"rmst difference"` (another Kaplan-Meier based test).
+#'   - `"survival difference"` (default): a Kaplan-Meier based test.
+#'   -`"rmst difference"`: another Kaplan-Meier based test.
 #' @param tau Desired milestone for `test = "survival difference"` or
 #'   `test = "rmst difference"`.
 #'
@@ -57,8 +57,16 @@ fixed_design_size_rmst <- function(enroll_rate,
                                    ratio = 1,
                                    alpha = 0.025,
                                    beta = 0.1,
-                                   test = "rmst difference",
+                                   test = c("survival_difference", "rmst_difference"),
                                    tau = NULL) {
+  test <- match.arg(test)
+
+  if (test == "survival_difference") {
+    test <- "survival difference"
+  } else if (test == "rmst_difference") {
+    test <- "rmst difference"
+  }
+
   gs_arm <- gs_create_arm(enroll_rate, fail_rate, ratio = ratio, total_time = analysis_time)
   arm0 <- gs_arm[["arm0"]]
   arm1 <- gs_arm[["arm1"]]
@@ -109,8 +117,8 @@ fixed_design_size_rmst <- function(enroll_rate,
 #' @param ratio Experimental:Control randomization ratio.
 #' @param alpha One-sided Type I error (strictly between 0 and 1).
 #' @param test A string specifies the type of statistical test.
-#'   Default is `"survival difference"` (a Kaplan-Meier based test).
-#'   One can also set it as `"rmst difference"` (another Kaplan-Meier based test).
+#'   - `"survival difference"` (default): a Kaplan-Meier based test.
+#'   - `"rmst difference"`: another Kaplan-Meier based test.
 #' @param tau Desired milestone for `test = "survival difference"` or
 #'   `test = "rmst difference"`.
 #'
@@ -136,8 +144,16 @@ fixed_design_power_rmst <- function(enroll_rate,
                                     analysis_time,
                                     ratio = 1,
                                     alpha = 0.025,
-                                    test = "rmst difference",
+                                    test = c("survival_difference", "rmst_difference"),
                                     tau = NULL) {
+  test <- match.arg(test)
+
+  if (test == "survival_difference") {
+    test <- "survival difference"
+  } else if (test == "rmst_difference") {
+    test <- "rmst difference"
+  }
+
   gs_arm <- gs_create_arm(enroll_rate, fail_rate, ratio = ratio, total_time = analysis_time)
   arm0 <- gs_arm[["arm0"]]
   arm1 <- gs_arm[["arm1"]]
diff --git a/tests/testthat/test-independent-rmst.R b/tests/testthat/test-independent-rmst.R
index bbf98cbf..a7b98243 100644
--- a/tests/testthat/test-independent-rmst.R
+++ b/tests/testthat/test-independent-rmst.R
@@ -12,7 +12,9 @@ test_that("given sample size, the output power arrives at the target", {
   )
 
   # output from gsDesign2
-  x1 <- fixed_design_size_rmst(enroll_rate, fail_rate, ratio = 1, analysis_time = 36, alpha = 0.025)
+  x1 <- gsDesign2:::fixed_design_size_rmst(enroll_rate, fail_rate,
+                                           ratio = 1, analysis_time = 36,
+                                           alpha = 0.025, test = "rmst_difference")
 
   # output from npsurvSS
   gs_arm <- gs_create_arm(enroll_rate, fail_rate, ratio = 1, total_time = 36)
@@ -28,6 +30,7 @@ test_that("given sample size, the output power arrives at the target", {
   expect_equal(x1$bound$probability, x2)
 })
 
+
 test_that("given power, the output sample size arrives at the target power", {
   # set enrollment rates
   enroll_rate <- tibble::tibble(stratum = "All", duration = 12, rate = 500 / 12)
@@ -42,7 +45,9 @@ test_that("given power, the output sample size arrives at the target power", {
   )
 
   # output from gsDesign2
-  x1 <- fixed_design_power_rmst(enroll_rate, fail_rate, analysis_time = 36)
+  x1 <- fixed_design_power_rmst(enroll_rate, fail_rate,
+                                analysis_time = 36,
+                                test = "rmst_difference")
 
   # output from npsurvSS
   gs_arm <- gs_create_arm(enroll_rate, fail_rate, ratio = 1, total_time = 36)