diff --git a/avoidable_admissions/data/validate.py b/avoidable_admissions/data/validate.py
index e1fe88e..952e1d8 100644
--- a/avoidable_admissions/data/validate.py
+++ b/avoidable_admissions/data/validate.py
@@ -20,7 +20,7 @@ class AdmittedCareEpisodeSchema(pa.SchemaModel):
     # visit_id is not part of the data spec but is used here as a unique row identifier
     # Use `df["visit_id"] = df.reset_index(drop=True).index`
 
-    visit_id: Series[np.int64] = pa.Field(nullable=False, unique=True)
+    visit_id: Series[str] = pa.Field(nullable=False, unique=True, coerce=True)
 
     # Ensure this has been pseudonymised appropriately.
     patient_id: Series[str] = pa.Field(nullable=False)
@@ -219,7 +219,7 @@ class Config:
 
 class EmergencyCareEpisodeSchema(pa.SchemaModel):
 
-    visit_id: Series[np.int64] = pa.Field(nullable=False, unique=True)
+    visit_id: Series[str] = pa.Field(nullable=False, unique=True, coerce=True)
 
     patient_id: Series[str] = pa.Field(nullable=False)