diff --git a/lib/bundler/definition.rb b/lib/bundler/definition.rb
index d9b5660e1f..80a2eb4714 100644
--- a/lib/bundler/definition.rb
+++ b/lib/bundler/definition.rb
@@ -887,8 +887,6 @@ module Bundler
       converged = []
       deps = []
 
-      @specs_that_changed_sources = []
-
       specs.each do |s|
         name = s.name
         dep = @dependencies.find {|d| s.satisfies?(d) }
@@ -897,7 +895,6 @@ module Bundler
         if dep
           gemfile_source = dep.source || default_source
 
-          @specs_that_changed_sources << s if gemfile_source != lockfile_source
           deps << dep if !dep.source || lockfile_source.include?(dep.source)
           @gems_to_unlock << name if lockfile_source.include?(dep.source) && lockfile_source != gemfile_source
 
@@ -979,7 +976,6 @@ module Bundler
         source_requirements["bundler"] = sources.metadata_source # needs to come last to override
       end
 
-      verify_changed_sources!
       source_requirements
     end
 
@@ -987,14 +983,6 @@ module Bundler
       sources.default_source
     end
 
-    def verify_changed_sources!
-      @specs_that_changed_sources.each do |s|
-        if s.source.specs.search(s.name).empty?
-          raise GemNotFound, "Could not find gem '#{s.name}' in #{s.source}"
-        end
-      end
-    end
-
     def requested_groups
       values = groups - Bundler.settings[:without] - @optional_groups + Bundler.settings[:with]
       values &= Bundler.settings[:only] unless Bundler.settings[:only].empty?
diff --git a/lib/bundler/lazy_specification.rb b/lib/bundler/lazy_specification.rb
index 8669e021c2..74f7da188f 100644
--- a/lib/bundler/lazy_specification.rb
+++ b/lib/bundler/lazy_specification.rb
@@ -27,11 +27,18 @@ module Bundler
       @dependencies  = []
       @required_ruby_version = Gem::Requirement.default
       @required_rubygems_version = Gem::Requirement.default
-      @platform      = platform || Gem::Platform::RUBY
-      @source        = source
+      @platform = platform || Gem::Platform::RUBY
+
+      @original_source = source
+      @source = source
+
       @force_ruby_platform = default_force_ruby_platform
     end
 
+    def source_changed?
+      @original_source != source
+    end
+
     def full_name
       @full_name ||= if platform == Gem::Platform::RUBY
         "#{@name}-#{@version}"
diff --git a/lib/bundler/resolver/base.rb b/lib/bundler/resolver/base.rb
index 3f2436672a..4c2aed32de 100644
--- a/lib/bundler/resolver/base.rb
+++ b/lib/bundler/resolver/base.rb
@@ -107,6 +107,10 @@ module Bundler
       def build_base_requirements
         base_requirements = {}
         @base.each do |ls|
+          if ls.source_changed? && ls.source.specs.search(ls.name).empty?
+            raise GemNotFound, "Could not find gem '#{ls.name}' in #{ls.source}"
+          end
+
           req = Gem::Requirement.new(ls.version)
           base_requirements[ls.name] = req
         end
diff --git a/spec/bundler/resolver/basic_spec.rb b/spec/bundler/resolver/basic_spec.rb
index f152907d35..0d3eadbaf8 100644
--- a/spec/bundler/resolver/basic_spec.rb
+++ b/spec/bundler/resolver/basic_spec.rb
@@ -238,7 +238,7 @@ RSpec.describe "Resolving" do
     it "resolves foo only to latest patch - changing dependency declared case" do
       # bar is locked AND a declared dependency in the Gemfile, so it will not move, and therefore
       # foo can only move up to 1.4.4.
-      @base << build_spec("bar", "2.0.3").first
+      @base << Bundler::LazySpecification.new("bar", Gem::Version.new("2.0.3"), nil)
       should_conservative_resolve_and_include :patch, ["foo"], %w[foo-1.4.4 bar-2.0.3]
     end