diff --git a/CMakeLists.txt b/CMakeLists.txt
index 773adae..2f7e9c7 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -101,6 +101,7 @@ set( fc_sources
      src/thread/spin_lock.cpp 
      src/thread/spin_yield_lock.cpp 
      src/thread/mutex.cpp
+     src/thread/non_preemptable_scope_check.cpp
      src/asio.cpp
      src/string.cpp
      src/shared_ptr.cpp
diff --git a/include/fc/thread/non_preemptable_scope_check.hpp b/include/fc/thread/non_preemptable_scope_check.hpp
new file mode 100644
index 0000000..e52a1b9
--- /dev/null
+++ b/include/fc/thread/non_preemptable_scope_check.hpp
@@ -0,0 +1,35 @@
+#pragma once
+#include <fc/time.hpp>
+#include <fc/thread/spin_yield_lock.hpp>
+
+// This file defines a macro:
+//   ASSERT_TASK_NOT_PREEMPTED()
+// This macro is used to declare that the current scope is not expected to yield.
+// If the task does yield, it will generate an assertion.
+//
+// Use this when you you're coding somethign that must not yield, and you think you
+// have done it (just by virtue of the fact that you don't think you've called any
+// functions that could yield).  This will help detect when your assumptions are
+// wrong and you accidentally call something that yields.
+//
+// This has no cost in release mode, and is extremely cheap in debug mode.
+
+#define FC_NON_PREEMPTABLE_SCOPE_COMBINE_HELPER(x,y) x ## y
+#define FC_NON_PREEMPTABLE_SCOPE_COMBINE(x,y) FC_NON_PREEMPTABLE_SCOPE_COMBINE_HELPER(x,y)
+
+#ifdef NDEBUG
+# define ASSERT_TASK_NOT_PREEMPTED() do {} while (0)
+#else
+# define ASSERT_TASK_NOT_PREEMPTED() fc::non_preemptable_scope_check FC_NON_PREEMPTABLE_SCOPE_COMBINE(scope_checker_, __LINE__)
+
+namespace fc 
+{ 
+  class non_preemptable_scope_check
+  {
+  public:
+    non_preemptable_scope_check();
+    ~non_preemptable_scope_check();
+  };
+} // namespace fc
+#endif
+
diff --git a/include/fc/thread/thread.hpp b/include/fc/thread/thread.hpp
index e6f01c8..1b99b80 100644
--- a/include/fc/thread/thread.hpp
+++ b/include/fc/thread/thread.hpp
@@ -119,6 +119,9 @@ namespace fc {
       friend class promise_base;
       friend class thread_d;
       friend class mutex;
+#ifndef NDEBUG
+      friend class non_preemptable_scope_check;
+#endif
       friend void yield();
       friend void usleep(const microseconds&);
       friend void sleep_until(const time_point&);
diff --git a/src/thread/non_preemptable_scope_check.cpp b/src/thread/non_preemptable_scope_check.cpp
new file mode 100644
index 0000000..574bdd4
--- /dev/null
+++ b/src/thread/non_preemptable_scope_check.cpp
@@ -0,0 +1,20 @@
+#ifndef NDEBUG
+#include <fc/thread/non_preemptable_scope_check.hpp>
+#include <fc/thread/thread.hpp>
+#include "thread_d.hpp"
+
+
+namespace fc 
+{
+  non_preemptable_scope_check::non_preemptable_scope_check()
+  {
+    ++thread::current().my->non_preemptable_scope_count;
+  }
+
+  non_preemptable_scope_check::~non_preemptable_scope_check()
+  {
+    assert(thread::current().my->non_preemptable_scope_count > 0);
+    --thread::current().my->non_preemptable_scope_count;
+  }
+} // fc
+#endif
\ No newline at end of file
diff --git a/src/thread/thread_d.hpp b/src/thread/thread_d.hpp
index f533dd7..b40c096 100644
--- a/src/thread/thread_d.hpp
+++ b/src/thread/thread_d.hpp
@@ -27,6 +27,9 @@ namespace fc {
              ready_head(0),
              ready_tail(0),
              blocked(0)
+#ifndef NDEBUG
+             ,non_preemptable_scope_count(0)
+#endif
             { 
               static boost::atomic<int> cnt(0);
               name = fc::string("th_") + char('a'+cnt++); 
@@ -84,8 +87,9 @@ namespace fc {
            fc::context*             ready_tail;
 
            fc::context*             blocked;
-
-
+#ifndef NDEBUG
+           unsigned                 non_preemptable_scope_count;
+#endif
 
 #if 0
            void debug( const fc::string& s ) {
@@ -284,6 +288,7 @@ namespace fc {
             *   have it wait for something to do.
             */
            bool start_next_fiber( bool reschedule = false ) {
+              assert(non_preemptable_scope_count == 0);
               check_for_timeouts();
               if( !current ) current = new fc::context( &fc::thread::current() );
 
diff --git a/tests/task_cancel.cpp b/tests/task_cancel.cpp
index c0c76f6..07c107e 100644
--- a/tests/task_cancel.cpp
+++ b/tests/task_cancel.cpp
@@ -4,6 +4,38 @@
 #include <fc/thread/thread.hpp>
 #include <fc/log/logger.hpp>
 #include <fc/exception/exception.hpp>
+#include <fc/thread/non_preemptable_scope_check.hpp>
+
+BOOST_AUTO_TEST_CASE( test_non_preemptable_assertion )
+{
+  return; // this isn't a real test, because the thing it tries to test works by asserting, not by throwing
+  fc::usleep(fc::milliseconds(10)); // this should not assert
+  {
+    ASSERT_TASK_NOT_PREEMPTED();
+    fc::usleep(fc::seconds(1)); // this should assert
+  }
+
+  {
+    ASSERT_TASK_NOT_PREEMPTED();
+    {
+      ASSERT_TASK_NOT_PREEMPTED();
+      fc::usleep(fc::seconds(1)); // this should assert
+    }
+  }
+
+  {
+    ASSERT_TASK_NOT_PREEMPTED();
+    {
+      ASSERT_TASK_NOT_PREEMPTED();
+      int i = 4;
+      i += 2;
+    }
+    fc::usleep(fc::seconds(1)); // this should assert
+  }
+
+  fc::usleep(fc::seconds(1));  // this should not assert
+  BOOST_TEST_PASSPOINT();
+}
 
 BOOST_AUTO_TEST_CASE( cancel_an_active_task )
 {