diff --git a/os/common/oslib/include/chheap.h b/os/common/oslib/include/chheap.h
index 7c85efe7f..6cce85b3c 100644
--- a/os/common/oslib/include/chheap.h
+++ b/os/common/oslib/include/chheap.h
@@ -173,7 +173,7 @@ static inline void *chHeapAlloc(memory_heap_t *heapp, size_t size) {
  */
 static inline size_t chHeapGetSize(const void *p) {
 
-  return ((heap_header_t *)p)->used.size;
+  return ((heap_header_t *)p - 1U)->used.size;
 }
 
 #endif /* CH_CFG_USE_HEAP == TRUE */
diff --git a/readme.txt b/readme.txt
index 5cf5c6b86..ac083134d 100644
--- a/readme.txt
+++ b/readme.txt
@@ -155,6 +155,8 @@
        Updated the various platform.mk implementing "smart build" mode.
 - HAL: Fixed DAC CH2 marked as not present in STM32F091 registry (bug #905)
        (backported to 17.6.4 and 16.1.10).
+- LIB: Fixed chHeapGetSize value is not obtained from the header (bug #904)
+       (backported to 17.6.4).
 - HAL: Fixed ADC does not build on STM32F030 (bug #903)(backported to 17.6.4).
 - LIB: Fixed typo for function evtStop (bug #897)(backported to 17.6.4
        and 16.1.10).