diff --git a/compiler/rustc_parse/src/lexer/mod.rs b/compiler/rustc_parse/src/lexer/mod.rs
index b5b34c7338d88..b2604ea690851 100644
--- a/compiler/rustc_parse/src/lexer/mod.rs
+++ b/compiler/rustc_parse/src/lexer/mod.rs
@@ -532,10 +532,15 @@ impl<'a> StringReader<'a> {
             if let Err(err) = result {
                 let span_with_quotes =
                     self.mk_sp(content_start - BytePos(1), content_end + BytePos(1));
+                let (start, end) = (range.start as u32, range.end as u32);
+                let lo = content_start + BytePos(start);
+                let hi = lo + BytePos(end - start);
+                let span = self.mk_sp(lo, hi);
                 emit_unescape_error(
                     &self.sess.span_diagnostic,
                     lit_content,
                     span_with_quotes,
+                    span,
                     mode,
                     range,
                     err,
diff --git a/compiler/rustc_parse/src/lexer/unescape_error_reporting.rs b/compiler/rustc_parse/src/lexer/unescape_error_reporting.rs
index 47d317f918865..6a890be36956a 100644
--- a/compiler/rustc_parse/src/lexer/unescape_error_reporting.rs
+++ b/compiler/rustc_parse/src/lexer/unescape_error_reporting.rs
@@ -13,6 +13,8 @@ pub(crate) fn emit_unescape_error(
     lit: &str,
     // full span of the literal, including quotes
     span_with_quotes: Span,
+    // interior span of the literal, without quotes
+    span: Span,
     mode: Mode,
     // range of the error inside `lit`
     range: Range<usize>,
@@ -26,13 +28,6 @@ pub(crate) fn emit_unescape_error(
         range,
         error
     );
-    let span = {
-        let Range { start, end } = range;
-        let (start, end) = (start as u32, end as u32);
-        let lo = span_with_quotes.lo() + BytePos(start + 1);
-        let hi = lo + BytePos(end - start);
-        span_with_quotes.with_lo(lo).with_hi(hi)
-    };
     let last_char = || {
         let c = lit[range.clone()].chars().rev().next().unwrap();
         let span = span.with_lo(span.hi() - BytePos(c.len_utf8() as u32));
diff --git a/src/test/ui/attributes/key-value-non-ascii.rs b/src/test/ui/attributes/key-value-non-ascii.rs
new file mode 100644
index 0000000000000..96c77f4f865e4
--- /dev/null
+++ b/src/test/ui/attributes/key-value-non-ascii.rs
@@ -0,0 +1,5 @@
+#![feature(rustc_attrs)]
+
+#[rustc_dummy = b"ffi.rs"] //~ ERROR byte constant must be ASCII
+                          //~| ERROR byte constant must be ASCII
+fn main() {}
diff --git a/src/test/ui/attributes/key-value-non-ascii.stderr b/src/test/ui/attributes/key-value-non-ascii.stderr
new file mode 100644
index 0000000000000..43df0e453bb04
--- /dev/null
+++ b/src/test/ui/attributes/key-value-non-ascii.stderr
@@ -0,0 +1,14 @@
+error: byte constant must be ASCII. Use a \xHH escape for a non-ASCII byte
+  --> $DIR/key-value-non-ascii.rs:3:19
+   |
+LL | #[rustc_dummy = b"ffi.rs"]
+   |                   ^
+
+error: byte constant must be ASCII. Use a \xHH escape for a non-ASCII byte
+  --> $DIR/key-value-non-ascii.rs:3:21
+   |
+LL | #[rustc_dummy = b"ffi.rs"]
+   |                     ^^^
+
+error: aborting due to 2 previous errors
+