diff --git a/build_tests/simple/new_dep b/build_tests/simple/new_dep
new file mode 100644
index 0000000..bf05ff7
--- /dev/null
+++ b/build_tests/simple/new_dep
@@ -0,0 +1 @@
+# This file is referenced by cache_debugging.rb
diff --git a/lib/rscons/cache.rb b/lib/rscons/cache.rb
index d7f3502..aee4243 100644
--- a/lib/rscons/cache.rb
+++ b/lib/rscons/cache.rb
@@ -282,6 +282,16 @@ module Rscons
       @cache["directories"].keys
     end
 
+    # Return a file's checksum, or the previously calculated checksum for
+    # the same file.
+    #
+    # @param file [String] The file name.
+    #
+    # @return [String] The file's checksum.
+    def lookup_checksum(file)
+      @lookup_checksums[file] || calculate_checksum(file)
+    end
+
     private
 
     # Return a String key based on the target name to use in the on-disk cache.
@@ -312,16 +322,6 @@ module Rscons
       @lookup_checksums = {}
     end
 
-    # Return a file's checksum, or the previously calculated checksum for
-    # the same file.
-    #
-    # @param file [String] The file name.
-    #
-    # @return [String] The file's checksum.
-    def lookup_checksum(file)
-      @lookup_checksums[file] || calculate_checksum(file)
-    end
-
     # Calculate and return a file's checksum.
     #
     # @param file [String] The file name.
diff --git a/lib/rscons/environment.rb b/lib/rscons/environment.rb
index c1a3471..5f26a81 100644
--- a/lib/rscons/environment.rb
+++ b/lib/rscons/environment.rb
@@ -341,6 +341,7 @@ module Rscons
           cache.clear_checksum_cache!
 
           if job
+            validate_user_deps(job[:target], @user_deps[job[:target]], cache)
             result = run_builder(job[:builder],
                                  job[:target],
                                  job[:sources],
@@ -1107,5 +1108,25 @@ module Rscons
       end
       deps
     end
+
+    # Ensures that user dependencies exist with valid checksums in the
+    # cache.  Raises an exception if any dependency is invalid.
+    #
+    # @param target [String]
+    #   Target to be built
+    # @param deps [Array<String>, nil]
+    #   User dependencies of the target
+    # @param cache [Cache]
+    #   Rscons cache instance
+    #
+    # @return [void]
+    def validate_user_deps(target, user_deps, cache)
+      return if user_deps.nil?
+      user_deps.each do |dep|
+        if cache.lookup_checksum(dep) == ""
+          raise "User dependency #{dep} of target #{target} is invalid"
+        end
+      end
+    end
   end
 end
diff --git a/spec/build_tests_spec.rb b/spec/build_tests_spec.rb
index 55ae5ec..fcd0b45 100644
--- a/spec/build_tests_spec.rb
+++ b/spec/build_tests_spec.rb
@@ -417,14 +417,14 @@ EOF
     expect(result.stderr).to eq ""
     expect(lines(result.stdout)).to eq(["LD simple.exe"])
 
-    File.unlink("program.ld")
-    result = run_test(rsconsfile: "user_dependencies.rb")
-    expect(result.stderr).to eq ""
-    expect(lines(result.stdout)).to eq ["LD simple.exe"]
-
     result = run_test(rsconsfile: "user_dependencies.rb")
     expect(result.stderr).to eq ""
     expect(result.stdout).to eq ""
+
+    File.unlink("program.ld")
+    result = run_test(rsconsfile: "user_dependencies.rb")
+    expect(result.stderr).to match /User dependency program\.ld of target simple\.exe is invalid/
+    expect(result.status).to_not eq 0
   end
 
   unless ENV["omit_gdc_tests"]