diff --git a/app/models/pet.rb b/app/models/pet.rb
index e03b0273..a4299a32 100644
--- a/app/models/pet.rb
+++ b/app/models/pet.rb
@@ -37,20 +37,35 @@ class Pet < ApplicationRecord
     raise UnexpectedDataFormat unless pet_data[:color_id]
     raise UnexpectedDataFormat unless pet_data[:body_id]
 
+    has_alt_style = pet_data[:alt_style].present?
+
     self.pet_type = PetType.find_or_initialize_by(
       species_id: pet_data[:species_id].to_i,
       color_id: pet_data[:color_id].to_i
     )
-    self.pet_type.body_id = pet_data[:body_id]
     self.pet_type.origin_pet = self
 
-    pet_state_biology = pet_data[:alt_style] ?
-      pet_data[:original_biology] : pet_data[:biology_by_zone]
+    # With an alt style, `body_id` in the biology data refers to the body ID of
+    # the *alt* style, not the usual pet type. (We have `original_biology` for
+    # *some* of the pet type's situation, but not it's body ID!)
+    #
+    # So, in the alt style case, don't update `body_id` - but if this is our
+    # first time seeing this pet type and it doesn't *have* a `body_id` yet,
+    # let's not be creating it without one. We'll need to model it without the
+    # alt style first. (I don't bother with a clear error message though 😅)
+    self.pet_type.body_id = pet_data[:body_id] unless has_alt_style
+    if self.pet_type.body_id.nil?
+      raise UnexpectedDataFormat,
+        "can't process alt style on first occurrence of pet type"
+    end
+
+    pet_state_biology = has_alt_style ? pet_data[:original_biology] :
+      pet_data[:biology_by_zone]
     raise UnexpectedDataFormat if pet_state_biology.empty?
     pet_state_biology[0] = nil # remove effects if present
     @pet_state = self.pet_type.add_pet_state_from_biology! pet_state_biology
 
-    if pet_data[:alt_style]
+    if has_alt_style
       raise UnexpectedDataFormat unless pet_data[:alt_color]
       raise UnexpectedDataFormat if pet_data[:biology_by_zone].empty?