diff --git a/lib/forminate.rb b/lib/forminate.rb
index 5bdaa1b..2e1cac1 100644
--- a/lib/forminate.rb
+++ b/lib/forminate.rb
@@ -122,8 +122,9 @@ def build_associations(attributes)
     association_names.each do |association_name|
       association_builder = AssociationBuilder.new(association_name, attributes)
       association = association_builder.build
+      keys_for_cleanup = association_builder.attribute_keys_for_cleanup.dup
       attributes.delete_if do |key, _|
-        association_builder.attribute_keys_for_cleanup.include?(key.to_sym)
+        keys_for_cleanup.include?(key.to_sym)
       end
       instance_variable_set("@#{association_name}".to_sym, association)
     end
diff --git a/lib/forminate/association_builder.rb b/lib/forminate/association_builder.rb
index 120a693..98ed942 100644
--- a/lib/forminate/association_builder.rb
+++ b/lib/forminate/association_builder.rb
@@ -16,6 +16,8 @@ def build
     end
 
     def attribute_keys_for_cleanup
+      return [] unless nested_attributes.present?
+
       prefixed_attributes.keys.push(name)
     end
 
@@ -62,7 +64,7 @@ def unprefixed_attributes
     end
 
     def nested_attributes
-      attrs[name]
+      attrs[name] if attrs[name].is_a?(Hash)
     end
   end
 end