diff --git a/DESCRIPTION b/DESCRIPTION
index c2583ed..4ed8be8 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -1,6 +1,6 @@
 Package: yulab.utils
 Title: Supporting Functions for Packages Maintained by 'YuLab-SMU'
-Version: 0.1.7
+Version: 0.1.7.001
 Authors@R: c(person("Guangchuang", "Yu", email = "guangchuangyu@gmail.com", role = c("aut", "cre"), comment = c(ORCID = "0000-0002-6485-8781")))
 Description: Miscellaneous functions commonly used by 'YuLab-SMU'.
 Imports:
diff --git a/NEWS.md b/NEWS.md
index 531628e..3106c82 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,3 +1,7 @@
+# yulab.utils 0.1.7.001
+
++ bug fixed in `pkg_ref()` (2024-08-26, Mon)
+
 # yulab.utils 0.1.7
 
 + `has_internet()` for testing internet connection (2024-08-26, Mon)
diff --git a/R/yulab-msg.R b/R/yulab-msg.R
index 2df2e1c..928f6e1 100644
--- a/R/yulab-msg.R
+++ b/R/yulab-msg.R
@@ -93,7 +93,7 @@ pkg_ref <- function(pkgname) {
 
     ref <- refs[pkgname]
 
-    if (is.null(ref)) return(NULL)
+    if (is.null(ref) || is.na(ref)) return(NULL)
 
     msg <- "Please cite:\n\n" 
     str_wrap(paste0(msg, refs, "\n"))