diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 456bc25f32..552b586250 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -616,6 +616,9 @@ static char * deepestStack = (char *) -1; /* for measuring stack usage */
 
 Expr evalExpr2(EvalState & state, Expr e)
 {
+    /* When changing this function, make sure that you don't cause a
+       (large) increase in stack consumption! */
+    
     char x;
     if (&x < deepestStack) deepestStack = &x;
     
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 420911a876..dabbaa3237 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -12,6 +12,7 @@ namespace nix {
 
 MakeError(EvalError, Error)
 MakeError(AssertionError, EvalError)
+MakeError(ThrownError, AssertionError)
 MakeError(Abort, EvalError)
 MakeError(TypeError, EvalError)
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index aca5f5856d..c7fbca0bb5 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -269,6 +269,14 @@ static Expr prim_abort(EvalState & state, const ATermVector & args)
 }
 
 
+static Expr prim_throw(EvalState & state, const ATermVector & args)
+{
+    PathSet context;
+    throw ThrownError(format("user-thrown exception: `%1%'") %
+        evalString(state, args[0], context));
+}
+
+
 /* Return an environment variable.  Use with care. */
 static Expr prim_getEnv(EvalState & state, const ATermVector & args)
 {
@@ -878,6 +886,7 @@ void EvalState::addPrimOps()
     addPrimOp("isNull", 1, prim_isNull);
     addPrimOp("dependencyClosure", 1, prim_dependencyClosure);
     addPrimOp("abort", 1, prim_abort);
+    addPrimOp("throw", 1, prim_throw);
     addPrimOp("__getEnv", 1, prim_getEnv);
 
     addPrimOp("relativise", 2, prim_relativise);