diff --git a/core/lib/rom/schema.rb b/core/lib/rom/schema.rb
index 3777c60a2..afd926fa1 100644
--- a/core/lib/rom/schema.rb
+++ b/core/lib/rom/schema.rb
@@ -220,19 +220,16 @@ def to_h
     # @raise KeyError
     #
     # @api public
-    def [](key, src = name.to_sym)
-      attr =
-        if count_index[key].equal?(1)
-          name_index[key]
-        else
-          source_index[src][key]
-        end
-
-      unless attr
+    def [](key, src = nil)
+      if count_index[key] == 0
         raise(KeyError, "#{key.inspect} attribute doesn't exist in #{src} schema")
+      elsif count_index[key] > 1 && src.nil?
+        raise(KeyError, "#{key.inspect} attribute is not unique") if count_index[key] > 1
+      elsif src
+        source_index[src][key]
+      else
+        name_index[key]
       end
-
-      attr
     end
 
     # Project a schema to include only specified attributes
@@ -460,7 +457,9 @@ def set!(key, value)
 
     # @api private
     def count_index
-      map(&:name).map { |name| [name, count { |attr| attr.name == name }] }.to_h
+      reduce(Hash.new(0)) do |index, attr|
+        index.merge(attr.name => index[attr.name] + 1)
+      end
     end
 
     # @api private
diff --git a/core/spec/unit/rom/schema/accessing_attributes_spec.rb b/core/spec/unit/rom/schema/accessing_attributes_spec.rb
index 1e44a1df2..f4dfd150b 100644
--- a/core/spec/unit/rom/schema/accessing_attributes_spec.rb
+++ b/core/spec/unit/rom/schema/accessing_attributes_spec.rb
@@ -34,10 +34,6 @@
       define_schema(:tasks, id: :Integer, title: :String)
     end
 
-    it 'returns an attribute identified by its canonical name' do
-      expect(schema[:id]).to eql(define_attribute(:Integer, { name: :id }, source: :users))
-    end
-
     it 'returns an attribute identified by its canonical name when its unique' do
       expect(schema[:title]).to eql(define_attribute(:String, { name: :title }, source: :tasks))
     end
@@ -50,5 +46,9 @@
       expect { schema[:not_here] }.to raise_error(KeyError, /not_here/)
       expect { schema[:not_here, :tasks] }.to raise_error(KeyError, /not_here/)
     end
+
+    it 'raises KeyError when attribute name is not unique' do
+      expect { schema[:id] }.to raise_error(KeyError, /id/)
+    end
   end
 end