diff --git a/modules/libcom/src/osi/os/RTEMS/osdThread.c b/modules/libcom/src/osi/os/RTEMS/osdThread.c
index b23f66b8f..fdc31759f 100644
--- a/modules/libcom/src/osi/os/RTEMS/osdThread.c
+++ b/modules/libcom/src/osi/os/RTEMS/osdThread.c
@@ -366,7 +366,7 @@ threadMustCreate (const char *name,
 void epicsThreadMustJoin(epicsThreadId id)
 {
     rtems_id target_tid = (rtems_id)id, self_tid;
-    struct taskVar *v;
+    struct taskVar *v = 0;
 
     rtems_task_ident (RTEMS_SELF, 0, &self_tid);
 
@@ -375,10 +375,11 @@ void epicsThreadMustJoin(epicsThreadId id)
         rtems_task_get_note (target_tid, RTEMS_NOTEPAD_TASKVAR, &note);
         v = (void *)note;
     }
+    /* 'v' may be NULL if 'id' represents a non-EPICS thread other than _main_. */
 
-    if(!v->joinable) {
+    if(!v || !v->joinable) {
         if(epicsThreadGetIdSelf()==id) {
-            errlogPrintf("Warning: %s thread self-join of unjoinable\n", v->name);
+            errlogPrintf("Warning: %s thread self-join of unjoinable\n", v ? v->name : "non-EPICS thread");
 
         } else {
             /* try to error nicely, however in all likelyhood de-ref of
diff --git a/modules/libcom/test/epicsThreadTest.cpp b/modules/libcom/test/epicsThreadTest.cpp
index 08d7529fa..edaada914 100644
--- a/modules/libcom/test/epicsThreadTest.cpp
+++ b/modules/libcom/test/epicsThreadTest.cpp
@@ -172,10 +172,17 @@ MAIN(epicsThreadTest)
     unsigned int ncpus = epicsThreadGetCPUs();
     testDiag("System has %u CPUs", ncpus);
     testOk1(ncpus > 0);
+    testDiag("main() thread %p", epicsThreadGetIdSelf());
 
     testMyThread();
     testSelfJoin();
     testOkToBlock();
 
+    // attempt to self-join from a non-EPICS thread
+    // to make sure it does nothing as expected
+    eltc(0);
+    epicsThreadMustJoin(epicsThreadGetIdSelf());
+    eltc(1);
+
     return testDone();
 }