diff --git a/NEWS.md b/NEWS.md
index 63d85d5bd..a5f20e8df 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -44,6 +44,9 @@
   For the GNU Compiler Collection (gcc) this means v7 or newer; for clang it
   means v4 or newer — the same versions required by earlier MKVToolNix
   versions.
+* MKVToolNix now requires version 4 of fmtlib or later for the
+  `fmt::to_string` function. If a system-wide version is installed that's
+  older, the bundled copy will be used.
 
 
 # Version 46.0.0 "No Deeper Escape" 2020-05-01
diff --git a/ac/fmt.m4 b/ac/fmt.m4
index bcad451fc..691cde707 100644
--- a/ac/fmt.m4
+++ b/ac/fmt.m4
@@ -15,7 +15,7 @@ AC_CACHE_CHECK([fmt],[ac_cv_fmt],[
     #include <fmt/format.h>
     #include <fmt/ostream.h>
   ],[
-    fmt::format("{0:02}", 4254);
+    fmt::format("{0:02}", fmt::to_string(4254));
   ],[ac_cv_fmt=yes],[ac_cv_fmt=no])
 
   CXXFLAGS="$ac_save_CXXFLAGS"