diff --git a/include/erasurecode/erasurecode.h b/include/erasurecode/erasurecode.h
index 5f5e054..a17c45c 100644
--- a/include/erasurecode/erasurecode.h
+++ b/include/erasurecode/erasurecode.h
@@ -371,6 +371,7 @@ int liberasurecode_get_minimum_encode_size(int desc);
  *
  * @return fragment size - sizeof(fragment_header) + size
  *                         + frag_backend_metadata_size
+ *                         if an error, return value will be negative
  */
 int liberasurecode_get_fragment_size(int desc, int data_len);
 
diff --git a/src/erasurecode.c b/src/erasurecode.c
index 73091fd..31fc884 100644
--- a/src/erasurecode.c
+++ b/src/erasurecode.c
@@ -1221,6 +1221,8 @@ int liberasurecode_get_fragment_size(int desc, int data_len)
 {
     ec_backend_t instance = liberasurecode_backend_instance_get_by_desc(desc);
     // TODO: Create a common function to calculate fragment size also for preprocessing
+    if (NULL == instance)
+        return -EBACKENDNOTAVAIL;
     int aligned_data_len = get_aligned_data_size(instance, data_len);
     int size = (aligned_data_len / instance->args.uargs.k) + instance->common.backend_metadata_size;