diff --git a/src/libexpr/lexer.l b/src/libexpr/lexer.l
index 8855d794e..29ca327c1 100644
--- a/src/libexpr/lexer.l
+++ b/src/libexpr/lexer.l
@@ -209,7 +209,11 @@ or          { return OR_KW; }
 \#[^\r\n]*    /* single-line comments */
 \/\*([^*]|\*+[^*/])*\*+\/  /* long comments */
 
-{ANY}           return yytext[0];
+{ANY}       {
+              /* Don't return a negative number, as this will cause
+                 Bison to stop parsing without an error. */
+              return (unsigned char) yytext[0];
+            }
 
 }
 
diff --git a/tests/lang/parse-fail-uft8.nix b/tests/lang/parse-fail-uft8.nix
new file mode 100644
index 000000000..34948d48a
--- /dev/null
+++ b/tests/lang/parse-fail-uft8.nix
@@ -0,0 +1 @@
+123 é 4