diff --git a/CHANGES b/CHANGES
index 63be2ac1d5..83d562a66e 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,5 @@
+4500.	[bug]		Support modifier I64 in isc__print_printf. [RT #43526]
+
 4499.	[port]		MacOSX: silence deprecated function warning 
 			by using arc4random_stir() when available
 			instead of arc4random_addrandom(). [RT #43503]
diff --git a/lib/isc/print.c b/lib/isc/print.c
index d79a85a5ee..a6eb668d24 100644
--- a/lib/isc/print.c
+++ b/lib/isc/print.c
@@ -283,6 +283,16 @@ isc__print_printf(void (*emit)(char, void *), void *arg,
 			z = 1;
 			format++;
 			goto doint;
+#ifdef WIN32
+		case 'I':
+			/* Windows has I64 as a modifier for a quad. */
+			if (format[1] == '6' && format[2] == '4') {
+				q = 1;
+				format += 3;
+				goto doint;
+			}
+			continue;
+#endif
 		case 'n':
 		case 'i':
 		case 'd':