diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index a4812de0..3e955ea3 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);
 }
 
 
diff --git a/tests/lang/eval-okay-substring.exp b/tests/lang/eval-okay-substring.exp
index d936b7e9..6aace04b 100644
--- a/tests/lang/eval-okay-substring.exp
+++ b/tests/lang/eval-okay-substring.exp
@@ -1 +1 @@
-"ooxfoobarybarzobaabb"
+"ooxfoobarybarzobaabbc"
diff --git a/tests/lang/eval-okay-substring.nix b/tests/lang/eval-okay-substring.nix
index 184d7258..424af00d 100644
--- a/tests/lang/eval-okay-substring.nix
+++ b/tests/lang/eval-okay-substring.nix
@@ -17,3 +17,5 @@ substring 1 2 s
 + substring 3 0 s
 + "b"
 + substring 3 1 s
++ "c"
++ substring 5 10 "perl"