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-rw-r--r--src/libexpr/primops.cc2
1 files changed, 1 insertions, 1 deletions
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index a4812de06519..3e955ea3fe09 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -965,7 +965,7 @@ static void prim_substring(EvalState & state, Value * * args, Value & v)
 
     if (start < 0) throw EvalError("negative start position in `substring'");
 
-    mkString(v, string(s, start, len), context);
+    mkString(v, start >= s.size() ? "" : string(s, start, len), context);
 }