diff --git a/src/sds.c b/src/sds.c
index c52c14759..1c0ddd559 100644
--- a/src/sds.c
+++ b/src/sds.c
@@ -415,7 +415,7 @@ size_t sdsAllocSize(sds s) {
     char type = s[-1] & SDS_TYPE_MASK;
     /* SDS_TYPE_5 header doesn't contain the size of the allocation */
     if (type == SDS_TYPE_5) {
-        return s_malloc_size(sdsAllocPtr(s));
+        return s_malloc_usable_size(sdsAllocPtr(s));
     } else {
         return sdsHdrSize(type) + sdsalloc(s) + 1;
     }
diff --git a/src/sdsalloc.h b/src/sdsalloc.h
index 6fd076d9f..dfa8257eb 100644
--- a/src/sdsalloc.h
+++ b/src/sdsalloc.h
@@ -51,5 +51,6 @@
 #define s_trymalloc_usable ztrymalloc_usable
 #define s_tryrealloc_usable ztryrealloc_usable
 #define s_malloc_size zmalloc_size
+#define s_malloc_usable_size zmalloc_usable_size
 
 #endif