diff --git a/lib/AssemblyUtil/FastaToAssembly.py b/lib/AssemblyUtil/FastaToAssembly.py
index 3ffa54e..25b39b8 100644
--- a/lib/AssemblyUtil/FastaToAssembly.py
+++ b/lib/AssemblyUtil/FastaToAssembly.py
@@ -484,9 +484,9 @@ def _validate_mass_params(self, params):
 
         # validate object_metadata is a mapping if provided
         obj_metas = [i.get(_OBJ_META) for i in inputs]
-        for obj_meta in obj_metas:
+        for i, obj_meta in enumerate(obj_metas, start=1):
             if obj_meta is not None and type(obj_meta) != dict:
-                raise ValueError(f"{_OBJ_META} must be a mapping if provided")
+                raise ValueError(f"{_OBJ_META} must be a mapping if provided for entry #{i}")
 
     def _get_int(self, putative_int, name, minimum=1):
         if putative_int is not None:
diff --git a/test/FastaToAssembly_unit_test.py b/test/FastaToAssembly_unit_test.py
index aa7a048..e25420c 100644
--- a/test/FastaToAssembly_unit_test.py
+++ b/test/FastaToAssembly_unit_test.py
@@ -640,10 +640,12 @@ def test_import_fasta_mass_fail_min_contig_length():
 
 
 def test_import_fasta_mass_fail_invalid_obj_meta():
-    err1 = 'object_metadata must be a mapping if provided'
+    err1 = 'object_metadata must be a mapping if provided for entry #1'
     test_spec = [
         (err1,
          {'workspace_id': 3, 'inputs': [{'node': 'a', 'assembly_name': 'x', 'object_metadata': 'foo'}]}),
+        (err1,
+         {'workspace_id': 3, 'inputs': [{'node': 'a', 'assembly_name': 'x', 'object_metadata': []}]})
     ]
     _run_test_spec_fail(test_spec, mass=True)