diff --git a/src/EFCore.PG/Storage/Internal/NpgsqlTypeMappingSource.cs b/src/EFCore.PG/Storage/Internal/NpgsqlTypeMappingSource.cs
index e5719e76e..1a4a8b04c 100644
--- a/src/EFCore.PG/Storage/Internal/NpgsqlTypeMappingSource.cs
+++ b/src/EFCore.PG/Storage/Internal/NpgsqlTypeMappingSource.cs
@@ -785,7 +785,14 @@ static Type FindTypeToInstantiate(Type collectionType, Type elementType)
         EnumDefinition? enumDefinition;
         if (storeType is null)
         {
-            enumDefinition = _enumDefinitions.SingleOrDefault(m => m.ClrType == clrType);
+            var matches = _enumDefinitions.Where(m => m.ClrType == clrType).ToList();
+            if (matches.Count > 1)
+            {
+                throw new InvalidOperationException(
+                    $"Multiple enum definitions registered for CLR type '{clrType?.FullName}'. ");
+            }
+
+            enumDefinition = matches.FirstOrDefault();
 
             if (enumDefinition is null)
             {