diff --git a/checkin_notes b/checkin_notes
index c9b236d5ab..662b9905cf 100644
--- a/checkin_notes
+++ b/checkin_notes
@@ -6927,3 +6927,11 @@ David  7 Oct 2011
 
     lib/
         coproc.cpp,h
+
+David  7 Oct 2011
+    - client (Mac): there was no error check of the creation
+         of a memory-mapped file used for communication with an app.
+         Add a check, and don't start the job if it fails.
+
+    client/
+        app_start.cpp
diff --git a/client/app_start.cpp b/client/app_start.cpp
index 9a9f9a27ab..ed0013c7a4 100644
--- a/client/app_start.cpp
+++ b/client/app_start.cpp
@@ -841,6 +841,13 @@ int ACTIVE_TASK::start(bool first_time) {
             retval = create_shmem_mmap(
                 buf, sizeof(SHARED_MEM), (void**)&app_client_shm.shm
             );
+            if (retval) {
+                msg_printf(wup->project, MSG_INTERNAL_ERROR,
+                    "ACTIVE_TASK::start(): can't create memory-mapped file: %s",
+                    boincerror(retval)
+                );
+                return retval;
+            }
         } else {
             // Use shmget() shared memory
             retval = create_shmem(