diff --git a/lib/bundler/plugin/api/source.rb b/lib/bundler/plugin/api/source.rb
index b129576b7dc..55797d60043 100644
--- a/lib/bundler/plugin/api/source.rb
+++ b/lib/bundler/plugin/api/source.rb
@@ -287,6 +287,12 @@ def gem_install_dir
         def root
           Bundler.root
         end
+
+        # @private
+        # Returns true
+        def bundler_plugin_api_source?
+          true
+        end
       end
     end
   end
diff --git a/lib/bundler/rubygems_ext.rb b/lib/bundler/rubygems_ext.rb
index 53a153e560c..7293e7cfc7a 100644
--- a/lib/bundler/rubygems_ext.rb
+++ b/lib/bundler/rubygems_ext.rb
@@ -42,7 +42,11 @@ def source
     attr_writer :full_gem_path unless instance_methods.include?(:full_gem_path=)
 
     def full_gem_path
-      if source.respond_to?(:path) || source.is_a?(Bundler::Plugin::API::Source)
+      # this cannot check source.is_a?(Bundler::Plugin::API::Source)
+      # because that _could_ trip the autoload, and if there are unresolved
+      # gems at that time, this method could be called inside another require,
+      # thus raising with that constant being undefined. Better to check a method
+      if source.respond_to?(:path) || (source.respond_to?(:bundler_plugin_api_source?) && source.bundler_plugin_api_source?)
         Pathname.new(loaded_from).dirname.expand_path(source.root).to_s.untaint
       else
         rg_full_gem_path
diff --git a/spec/install/gemfile/gemspec_spec.rb b/spec/install/gemfile/gemspec_spec.rb
index 0b621754afd..e3c2164df87 100644
--- a/spec/install/gemfile/gemspec_spec.rb
+++ b/spec/install/gemfile/gemspec_spec.rb
@@ -192,6 +192,26 @@
     expect(@err).not_to match(/ahh/)
   end
 
+  it "allows the gemspec to activate other gems" do
+    # see https://github.com/bundler/bundler/issues/5409
+    #
+    # issue was caused by rubygems having an unresolved gem during a require,
+    # so emulate that
+    system_gems %w(rack-1.0.0 rack-0.9.1 rack-obama-1.0)
+
+    build_lib("foo", :path => bundled_app)
+    gemspec = bundled_app("foo.gemspec").read
+    bundled_app("foo.gemspec").open("w") do |f|
+      f.write "#{gemspec.strip}.tap { gem 'rack-obama'; require 'rack-obama' }"
+    end
+
+    install_gemfile! <<-G
+      gemspec
+    G
+
+    expect(the_bundle).to include_gem "foo 1.0"
+  end
+
   it "allows conflicts" do
     build_lib("foo", :path => tmp.join("foo")) do |s|
       s.version = "1.0.0"