diff --git a/src/H5Cimage.c b/src/H5Cimage.c
index dbf26d0352f..24fcafaabb3 100644
--- a/src/H5Cimage.c
+++ b/src/H5Cimage.c
@@ -1289,7 +1289,7 @@ H5C__decode_cache_image_header(const H5F_t *f, H5C_t *cache_ptr, const uint8_t *
     p = *buf;
 
     /* Ensure buffer has enough data for signature comparison */
-    if (H5_IS_BUFFER_OVERFLOW(p, H5C__MDCI_BLOCK_SIGNATURE_LEN, *buf + buf_size))
+    if (H5_IS_BUFFER_OVERFLOW(p, H5C__MDCI_BLOCK_SIGNATURE_LEN, *buf + buf_size - 1))
         HGOTO_ERROR(H5E_CACHE, H5E_OVERFLOW, FAIL, "Insufficient buffer size for signature");
 
     /* Check signature */