diff --git a/src/session_base.cpp b/src/session_base.cpp
index 12900df5..d4e9855c 100644
--- a/src/session_base.cpp
+++ b/src/session_base.cpp
@@ -230,27 +230,29 @@ void zmq::session_base_t::clean_pipes ()
 void zmq::session_base_t::terminated (pipe_t *pipe_)
 {
     // Drop the reference to the deallocated pipe if required.
-    zmq_assert (pipe == pipe_ || incomplete_pipes.size () > 0);
+    zmq_assert (pipe == pipe_ || terminating_pipes.count (pipe_) == 1);
 
     if (pipe == pipe_)
         // If this is our current pipe, remove it
         pipe = NULL;
     else
         // Remove the pipe from the detached pipes set
-        incomplete_pipes.erase (pipe_);
+        terminating_pipes.erase (pipe_);
 
     // If we are waiting for pending messages to be sent, at this point
     // we are sure that there will be no more messages and we can proceed
     // with termination safely.
-    if (pending && !pipe && incomplete_pipes.size () == 0)
+    if (pending && !pipe && terminating_pipes.size () == 0)
         proceed_with_term ();
 }
 
 void zmq::session_base_t::read_activated (pipe_t *pipe_)
 {
     // Skip activating if we're detaching this pipe
-    if (incomplete_pipes.size () > 0 && pipe_ != pipe)
+    if (pipe != pipe_) {
+        zmq_assert (terminating_pipes.count (pipe_) == 1);
         return;
+    }
 
     if (likely (engine != NULL))
         engine->activate_out ();
@@ -261,8 +263,10 @@ void zmq::session_base_t::read_activated (pipe_t *pipe_)
 void zmq::session_base_t::write_activated (pipe_t *pipe_)
 {
     // Skip activating if we're detaching this pipe
-    if (incomplete_pipes.size () > 0 && pipe_ != pipe)
+    if (pipe != pipe_) {
+        zmq_assert (terminating_pipes.count (pipe_) == 1);
         return;
+    }
 
     if (engine)
         engine->activate_in ();
@@ -411,7 +415,7 @@ void zmq::session_base_t::detached ()
         && addr->protocol != "pgm" && addr->protocol != "epgm") {
         pipe->hiccup ();
         pipe->terminate (false);
-        incomplete_pipes.insert (pipe);
+        terminating_pipes.insert (pipe);
         pipe = NULL;
     }
 
diff --git a/src/session_base.hpp b/src/session_base.hpp
index 7b9f3fc6..fa765ba3 100644
--- a/src/session_base.hpp
+++ b/src/session_base.hpp
@@ -105,7 +105,7 @@ namespace zmq
         zmq::pipe_t *pipe;
         
         //  This set is added to with pipes we are disconnecting, but haven't yet completed
-        std::set<pipe_t *> incomplete_pipes;
+        std::set<pipe_t *> terminating_pipes;
 
         //  This flag is true if the remainder of the message being processed
         //  is still in the in pipe.