diff --git a/src/f_string.rs b/src/f_string.rs
index 96051a3..75165b5 100644
--- a/src/f_string.rs
+++ b/src/f_string.rs
@@ -38,12 +38,12 @@ impl Printf<str> for FString {
 	#[inline(always)]
     fn printf_unsafe<A: AsRef<str>>(&mut self, fmt: A) {
         let tmp = U16CString::from_str(fmt).unwrap();
-		let new_len = tmp.len();
+		let mut tmp_bytes = tmp.into_vec_with_nul();
+		let new_len = tmp_bytes.len();
 		if new_len > self.0.cap as usize {
 			// TODO: use Result instead
 			panic!("New string is too long and no printf ffi binding has been provided");
 		}
-		let mut tmp_bytes = tmp.into_vec_with_nul();
 		tmp_bytes.resize(self.0.cap as usize, 0);
 		unsafe {
 			std::ptr::copy_nonoverlapping(