diff --git a/os/rt/src/chthreads.c b/os/rt/src/chthreads.c
index fcd6aefe3..db9939c74 100644
--- a/os/rt/src/chthreads.c
+++ b/os/rt/src/chthreads.c
@@ -280,7 +280,8 @@ thread_t *chThdCreateI(const thread_descriptor_t *tdp) {
 thread_t *chThdCreate(const thread_descriptor_t *tdp) {
   thread_t *tp;
 
-#if CH_CFG_USE_REGISTRY == TRUE
+#if (CH_CFG_USE_REGISTRY == TRUE) &&                                        \
+    (CH_DBG_ENABLE_STACK_CHECK == TRUE) || (CH_CFG_USE_DYNAMIC == TRUE)
   chDbgAssert(chRegFindThreadByWorkingArea(tdp->wbase) == NULL,
               "working area in use");
 #endif
@@ -329,7 +330,8 @@ thread_t *chThdCreateStatic(void *wsp, size_t size,
              MEM_IS_ALIGNED(size, PORT_STACK_ALIGN) &&
              (prio <= HIGHPRIO) && (pf != NULL));
 
-#if CH_CFG_USE_REGISTRY == TRUE
+#if (CH_CFG_USE_REGISTRY == TRUE) &&                                        \
+    (CH_DBG_ENABLE_STACK_CHECK == TRUE) || (CH_CFG_USE_DYNAMIC == TRUE)
   chDbgAssert(chRegFindThreadByWorkingArea(wsp) == NULL,
               "working area in use");
 #endif
diff --git a/readme.txt b/readme.txt
index 8afb89cd3..f1d5da336 100644
--- a/readme.txt
+++ b/readme.txt
@@ -163,6 +163,8 @@
        dependencies and configuration directories. This makes possible
        to have multiple non-conflicting makefiles in the same project.
        Updated the various platform.mk implementing "smart build" mode.
+- RT:  Fixed compile error with assertions enabled and dynamic extensions
+       disabled (bug #909)(backported to 17.6.4).
 - RT:  Fixed gcc7 implicit-fallthrough (bug #906)(backported to 17.6.4
        and 16.1.10).
 - HAL: Fixed gcc7 implicit-fallthrough (bug #906)(backported to 17.6.4