diff --git a/compiler/rustc_parse/src/lexer/mod.rs b/compiler/rustc_parse/src/lexer/mod.rs
index 4a638ec3f80..4bf870eb7ce 100644
--- a/compiler/rustc_parse/src/lexer/mod.rs
+++ b/compiler/rustc_parse/src/lexer/mod.rs
@@ -268,6 +268,9 @@ impl<'a> StringReader<'a> {
                 // tokens like `<<` from `rustc_lexer`, and then add fancier error recovery to it,
                 // as there will be less overall work to do this way.
                 let token = unicode_chars::check_for_substitution(self, start, c, &mut err);
+                if c == '\x00' {
+                    err.help("source files must contain UTF-8 encoded text, unexpected null bytes might occur when a different encoding is used");
+                }
                 err.emit();
                 token?
             }
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diff --git a/src/test/ui/parser/utf16-be-without-bom.rs b/src/test/ui/parser/utf16-be-without-bom.rs
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diff --git a/src/test/ui/parser/utf16-le-without-bom.rs b/src/test/ui/parser/utf16-le-without-bom.rs
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