diff --git a/src/array_string.rs b/src/array_string.rs
index 154b74c..65a8a02 100644
--- a/src/array_string.rs
+++ b/src/array_string.rs
@@ -99,7 +99,7 @@ impl<A: Array<Item=u8>> ArrayString<A> {
     pub fn push_str<'a>(&mut self, s: &'a str) -> Result<(), CapacityError<&'a str>> {
         use std::io::Write;
 
-        if self.len() + s.len() > self.capacity() {
+        if s.len() > self.capacity() - self.len() {
             return Err(CapacityError::new(s));
         }
         unsafe {