diff --git a/sys/src/ape/lib/fmt/fmt.c b/sys/src/ape/lib/fmt/fmt.c
index 976bf7ca2..51bf9884d 100644
--- a/sys/src/ape/lib/fmt/fmt.c
+++ b/sys/src/ape/lib/fmt/fmt.c
@@ -80,7 +80,7 @@ __fmtinstall(int c, Fmts f)
 {
 	Convfmt *p, *ep;
 
-	if(c<=0 || c>=65536)
+	if(c<=0 || c>Runemax)
 		return -1;
 	if(!f)
 		f = __badfmt;
diff --git a/sys/src/libc/fmt/fmt.c b/sys/src/libc/fmt/fmt.c
index 82ae36543..a1dee68d5 100644
--- a/sys/src/libc/fmt/fmt.c
+++ b/sys/src/libc/fmt/fmt.c
@@ -61,7 +61,7 @@ _fmtinstall(int c, Fmts f)
 {
 	Convfmt *p, *ep;
 
-	if(c<=0 || c>=65536)
+	if(c<=0 || c>Runemax)
 		return -1;
 	if(!f)
 		f = _badfmt;