diff --git a/src/documents/migrations/1054_customfieldinstance_value_monetary_amount_and_more.py b/src/documents/migrations/1054_customfieldinstance_value_monetary_amount_and_more.py
index 37d1aaeab..0bc135646 100644
--- a/src/documents/migrations/1054_customfieldinstance_value_monetary_amount_and_more.py
+++ b/src/documents/migrations/1054_customfieldinstance_value_monetary_amount_and_more.py
@@ -1,5 +1,6 @@
-# Generated by Django 5.1.1 on 2024-09-29 05:16
+# Generated by Django 5.1.1 on 2024-09-29 16:13
 
+import django.db.models.functions.comparison
 import django.db.models.functions.text
 from django.db import migrations
 from django.db import models
@@ -18,13 +19,22 @@ class Migration(migrations.Migration):
                 db_persist=True,
                 expression=models.Case(
                     models.When(
-                        then=django.db.models.functions.text.Substr(
-                            "value_monetary",
-                            1,
+                        then=django.db.models.functions.comparison.Cast(
+                            django.db.models.functions.text.Substr("value_monetary", 1),
+                            output_field=models.DecimalField(
+                                decimal_places=2,
+                                max_digits=125,
+                            ),
                         ),
                         value_monetary__regex="^\\d+",
                     ),
-                    default=django.db.models.functions.text.Substr("value_monetary", 4),
+                    default=django.db.models.functions.comparison.Cast(
+                        django.db.models.functions.text.Substr("value_monetary", 4),
+                        output_field=models.DecimalField(
+                            decimal_places=2,
+                            max_digits=125,
+                        ),
+                    ),
                     output_field=models.DecimalField(decimal_places=2, max_digits=125),
                 ),
                 output_field=models.DecimalField(decimal_places=2, max_digits=125),
diff --git a/src/documents/models.py b/src/documents/models.py
index 51c19c83e..e10de523d 100644
--- a/src/documents/models.py
+++ b/src/documents/models.py
@@ -23,6 +23,7 @@ if settings.AUDIT_LOG_ENABLED:
     from auditlog.registry import auditlog
 
 from django.db.models import Case
+from django.db.models.functions import Cast
 from django.db.models.functions import Substr
 from django_softdelete.models import SoftDeleteModel
 
@@ -929,10 +930,16 @@ class CustomFieldInstance(models.Model):
             # If the value starts with a number and no currency symbol, use the whole string
             models.When(
                 value_monetary__regex=r"^\d+",
-                then=Substr("value_monetary", 1),
+                then=Cast(
+                    Substr("value_monetary", 1),
+                    output_field=models.DecimalField(decimal_places=2, max_digits=125),
+                ),
             ),
             # If the value starts with a 3-char currency symbol, use the rest of the string
-            default=Substr("value_monetary", 4),
+            default=Cast(
+                Substr("value_monetary", 4),
+                output_field=models.DecimalField(decimal_places=2, max_digits=125),
+            ),
             output_field=models.DecimalField(decimal_places=2, max_digits=125),
         ),
         output_field=models.DecimalField(decimal_places=2, max_digits=125),