diff --git a/NEWS.md b/NEWS.md
index 5d8e96930..7ad7add99 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -44,9 +44,10 @@
   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.
+* MKVToolNix now requires version 6.1.0 of fmtlib or later for the
+  `fmt::to_string` function and bugfixes to the formatting of floating point
+  numbers. If a system-wide version is installed that's older, the bundled
+  copy will be used.
 * The bundled version of fmtlib has been updated to release 6.2.1.
 
 
diff --git a/ac/fmt.m4 b/ac/fmt.m4
index 691cde707..821631170 100644
--- a/ac/fmt.m4
+++ b/ac/fmt.m4
@@ -14,6 +14,10 @@ AC_CACHE_CHECK([fmt],[ac_cv_fmt],[
   AC_TRY_COMPILE([
     #include <fmt/format.h>
     #include <fmt/ostream.h>
+
+    #if !defined(FMT_VERSION) || (FMT_VERSION < 60100)
+    #error fmtlib is too old, need 6.1.0 or later
+    #endif
   ],[
     fmt::format("{0:02}", fmt::to_string(4254));
   ],[ac_cv_fmt=yes],[ac_cv_fmt=no])