diff --git a/spaceprime/cli.py b/spaceprime/cli.py
index a5dcdbe..bc139b3 100644
--- a/spaceprime/cli.py
+++ b/spaceprime/cli.py
@@ -81,7 +81,7 @@ def read_individuals(args, coords):
 
 
 def read_anc_pop_id(anc_pop_id):
-    if anc_pop_id[0] is not None and not len(anc_pop_id) > 1:
+    if anc_pop_id is not None and not len(anc_pop_id) > 1:
         if not os.path.exists(anc_pop_id[0]):
             raise FileNotFoundError(f"anc_pop_id file {anc_pop_id[0]} not found")
         else:
diff --git a/spaceprime/demography.py b/spaceprime/demography.py
index e79d24b..815a9b4 100644
--- a/spaceprime/demography.py
+++ b/spaceprime/demography.py
@@ -6,6 +6,7 @@
 from typing import Union, List, Optional
 import msprime
 from .utilities import calc_migration_matrix
+import warnings
 
 
 ## not sure if I should add a subclass or not. There are only two new functions, so might not be worth it.
@@ -270,6 +271,13 @@ def add_ancestral_populations(
     if not isinstance(anc_sizes, list):
         anc_sizes = [anc_sizes]
 
+    # set anc_id to None and throw a warning if anc_sizes is less than 2
+    if len(anc_sizes) < 2 and anc_id is not None:
+        warnings.warn(
+            "anc_sizes is less than two, but you provided a list of anc_id's. If you meant to setup more than one anc_size, provide a list of sizes to anc_size. Otherwise, you're fine."
+        )
+        anc_id = None
+
     # check merge time
     if isinstance(merge_time, list):
         raise ValueError("merge_time should be a single float or int, not a list.")