diff --git a/R/check_balance.R b/R/check_balance.R
index a571887..925bc47 100644
--- a/R/check_balance.R
+++ b/R/check_balance.R
@@ -1,8 +1,8 @@
 #' @title Check the variable balance between the probability and non-probability samples
 #'
-#' @param x Formula specifying variables to check
-#' @param object Object of `nonprobsvy` class
-#' @param dig Number of digits for rounding (default = 2)
+#' @param x formula specifying variables to check
+#' @param object object of `nonprob` class
+#' @param dig number of digits for rounding (default = 2)
 #'
 #' @importFrom stats aggregate
 #' @importFrom survey svytotal
@@ -51,8 +51,8 @@ check_balance.nonprob <- function(x, object, dig = 2) {
     stop("The `x` argument must be a formula.")
   }
 
-  if (missing(object) || is.null(object)) {
-    stop("The `object` argument is required.")
+  if (missing(object) || is.null(object) || !inherits(object, "nonprob")) {
+    stop("The `object` argument of class `nonprob` is required.")
   }
 
   if (object$estimator == "mi") {
diff --git a/README.Rmd b/README.Rmd
index 2587395..9e42cc4 100644
--- a/README.Rmd
+++ b/README.Rmd
@@ -406,7 +406,7 @@ p2 <- exp(-0.5+0.5*(x2-2)^2)/(1+exp(-0.5+0.5*(x2-2)^2))
 flag_bd1 <- rbinom(n = N, size = 1, prob = p1)
 flag_srs <- as.numeric(1:N %in% sample(1:N, size = n))
 base_w_srs <- N/n
-population <- data.frame(x1,x2,y1,y2,p1,p2,base_w_srs, flag_bd1, flag_srs)
+population <- data.frame(x1,x2,y1,y2,p1,p2,base_w_srs, flag_bd1, flag_srs, pop_size = N)
 base_w_bd <- N/sum(population$flag_bd1)
 ```
 
@@ -414,7 +414,8 @@ Declare `svydesign` object with `survey` package
 
 ```{r}
 sample_prob <- svydesign(ids= ~1, weights = ~ base_w_srs, 
-                         data = subset(population, flag_srs == 1))
+                         data = subset(population, flag_srs == 1),
+                         fpc = ~ pop_size)
 
 sample_prob
 ```
diff --git a/man/check_balance.Rd b/man/check_balance.Rd
index e51069e..e947315 100644
--- a/man/check_balance.Rd
+++ b/man/check_balance.Rd
@@ -7,11 +7,11 @@
 check_balance(x, object, dig)
 }
 \arguments{
-\item{x}{Formula specifying variables to check}
+\item{x}{formula specifying variables to check}
 
-\item{object}{Object of \code{nonprobsvy} class}
+\item{object}{object of \code{nonprob} class}
 
-\item{dig}{Number of digits for rounding (default = 2)}
+\item{dig}{number of digits for rounding (default = 2)}
 }
 \value{
 A \code{list} containing nonprobability totals, probability totals, and their differences