diff --git a/src/lib.rs b/src/lib.rs
index 19bf9dd..385a4c4 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -98,7 +98,8 @@ fn gethostname_impl() -> OsString {
 
     unsafe {
         // This call always fails with ERROR_MORE_DATA, because we pass NULL to
-        // get the required buffer size.
+        // get the required buffer size.  GetComputerNameExW then fills buffer_size with the size
+        // of the host name string plus a trailing zero byte.
         GetComputerNameExW(
             ComputerNamePhysicalDnsHostname,
             PWSTR::null(),
@@ -123,8 +124,9 @@ fn gethostname_impl() -> OsString {
         )
     }
     assert!(
-        buffer_size as usize == buffer.len(),
-        "GetComputerNameExW unexpectedly changed the buffer size"
+        // GetComputerNameExW returns the size _without_ the trailing zero byte on the second call
+        buffer_size as usize == buffer.len() - 1,
+        "GetComputerNameExW changed the buffer size unexpectedly"
     );
 
     let end = buffer.iter().position(|&b| b == 0).unwrap_or(buffer.len());