diff --git a/src/core/pp.c b/src/core/pp.c
index 0c9e81ce..618f7515 100644
--- a/src/core/pp.c
+++ b/src/core/pp.c
@@ -1066,7 +1066,7 @@ void janet_buffer_format(
                     if (form[2] == '\0')
                         janet_buffer_push_bytes(b, s, l);
                     else {
-                        if (l != (int32_t) strlen((const char *) s))
+                        if (l != (int32_t) strnlen((const char *) s, l))
                             janet_panic("string contains zeros");
                         if (!strchr(form, '.') && l >= 100) {
                             janet_panic("no precision and string is too long to be formatted");