diff --git a/kernel/bootstrap/context.rb b/kernel/bootstrap/context.rb
index f91d5bb..e9501b1 100644
--- a/kernel/bootstrap/context.rb
+++ b/kernel/bootstrap/context.rb
@@ -110,6 +110,11 @@ class BlockContext
 end
 
 class BlockEnvironment
+  def prepare
+    Ruby.primitive :block_prepare
+    raise PrimitiveFailure, "primitive failed"
+  end
+
   def call(*args)
     Ruby.primitive :block_call
     raise PrimitiveFailure, "primitive failed"
diff --git a/kernel/core/kernel.rb b/kernel/core/kernel.rb
index 4de46ff..cc88879 100644
--- a/kernel/core/kernel.rb
+++ b/kernel/core/kernel.rb
@@ -287,7 +287,9 @@ module Kernel
   module_function :proc
 
   def caller(start=1)
-    return MethodContext.current.sender.calling_hierarchy(start)
+    ctx = MethodContext.current.sender
+    ctx = ctx.sender if ctx.__kind_of__(BlockContext)
+    return ctx.calling_hierarchy(start)
   end
   module_function :caller
 
diff --git a/kernel/core/proc.rb b/kernel/core/proc.rb
index 7e56905..d1369a9 100644
--- a/kernel/core/proc.rb
+++ b/kernel/core/proc.rb
@@ -15,7 +15,7 @@ class Proc
   end
 
   def binding
-    Binding.setup @block.home
+    Binding.setup @block.prepare
   end
 
   #--
diff --git a/shotgun/lib/methctx.c b/shotgun/lib/methctx.c
index c912e57..4c5d7ce 100644
--- a/shotgun/lib/methctx.c
+++ b/shotgun/lib/methctx.c
@@ -106,6 +106,16 @@ void blokenv_call(STATE, cpu c, OBJECT self, int num_args) {
 
 }
 
+/* Prepares a block context like blockenv_call, but
+ * does not activate it. */
+OBJECT blokenv_prepare(STATE, cpu c, OBJECT self) {
+  OBJECT ctx;
+  cpu_flush_sp(c);
+  cpu_create_block_context(state, c, self, c->sp);
+  ctx = blokenv_get_home(self);
+  return ctx;
+}
+
 void methctx_reference(STATE, OBJECT ctx) {
   struct fast_context *fc;
 
diff --git a/shotgun/lib/methctx.h b/shotgun/lib/methctx.h
index 89fd237..47da1a2 100644
--- a/shotgun/lib/methctx.h
+++ b/shotgun/lib/methctx.h
@@ -4,6 +4,7 @@
 OBJECT blokctx_s_under_context(STATE, OBJECT ctx);
 OBJECT blokenv_s_under_context2(STATE, OBJECT cmethod, OBJECT ctx, OBJECT ctx_block);
 void blokenv_call(STATE, cpu c, OBJECT self, int num_args);
+OBJECT blokenv_prepare(STATE, cpu c, OBJECT self);
 
 OBJECT methctx_dup(STATE, OBJECT self);
 OBJECT methctx_dup_chain(STATE, OBJECT ctx, OBJECT *also);
diff --git a/shotgun/lib/primitives.rb b/shotgun/lib/primitives.rb
index b0760ca..16be067 100644
--- a/shotgun/lib/primitives.rb
+++ b/shotgun/lib/primitives.rb
@@ -584,6 +584,18 @@ class ShotgunPrimitives
     CODE
   end
 
+  defprim :block_prepare
+  def block_prepare
+    <<-CODE
+    ARITY(0);
+    OBJECT t1;
+    GUARD(BLOCKENV_P(msg->recv));
+
+    t1 = blokenv_prepare(state, c, msg->recv);
+    RET(t1);
+    CODE
+  end
+
   defprim :io_write
   def io_write
     <<-CODE