diff --git a/NEWS.md b/NEWS.md
index 42bf0164f0..fe872d718c 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -10,6 +10,10 @@
 
 1. Custom binary operators from the `lubridate` package now work with objects of class `IDate` as with a `Date` subclass, [#6839](https://github.com/Rdatatable/data.table/issues/6839). Thanks @emallickhossain for the report and @aitap for the fix.
 
+## NOTES
+
+1. Continued work to remove non-API C functions, [#6180](https://github.com/Rdatatable/data.table/issues/6180). Thanks Ivan Krylov for the PRs and for writing a clear and concise guide about the R API: https://aitap.codeberg.page/R-api/.
+
 # data.table [v1.17.0](https://github.com/Rdatatable/data.table/milestone/34)  (20 Feb 2025)
 
 ## POTENTIALLY BREAKING CHANGES
diff --git a/src/data.table.h b/src/data.table.h
index fd066c3b1d..af72145ae8 100644
--- a/src/data.table.h
+++ b/src/data.table.h
@@ -42,7 +42,12 @@
 /* we mean the encoding bits, not CE_NATIVE in a UTF-8 locale */
 #define IS_UTF8(x)  (getCharCE(x) == CE_UTF8)
 #define IS_LATIN(x) (getCharCE(x) == CE_LATIN1)
-#define IS_ASCII(x) (LEVELS(x) & 64) // API expected in R >= 4.5
+// TODO: remove the `R_SVN_VERSION` check when R 4.5.0 is released (circa Apr. 2025)
+#if R_VERSION < R_Version(4, 5, 0) || R_SVN_REVISION < 86789
+# define IS_ASCII(x) (LEVELS(x) & 64)
+#else
+# define IS_ASCII(x) (Rf_charIsASCII(x)) // no CE_ASCII
+#endif
 #define IS_TRUE(x)  (TYPEOF(x)==LGLSXP && LENGTH(x)==1 && LOGICAL(x)[0]==TRUE)
 #define IS_FALSE(x) (TYPEOF(x)==LGLSXP && LENGTH(x)==1 && LOGICAL(x)[0]==FALSE)
 #define IS_TRUE_OR_FALSE(x) (TYPEOF(x)==LGLSXP && LENGTH(x)==1 && LOGICAL(x)[0]!=NA_LOGICAL)