diff --git a/src/libexpr/lexer.l b/src/libexpr/lexer.l
index 74a9065a4..810503bdc 100644
--- a/src/libexpr/lexer.l
+++ b/src/libexpr/lexer.l
@@ -243,6 +243,14 @@ or          { return OR_KW; }
   return HPATH;
 }
 
+<PATH_START>{ANY} |
+<PATH_START><<EOF>> {
+  /* This should be unreachable: PATH_START is only entered after matching
+     PATH_SEG or HPATH_START, and we rewind to re-parse those same patterns.
+     This rule exists to satisfy flex's %option nodefault requirement. */
+  unreachable();
+}
+
 {PATH} {
   if (yytext[yyleng-1] == '/')
     PUSH_STATE(INPATH_SLASH);