diff --git a/core/lib/Drupal/Core/Extension/InfoParserDynamic.php b/core/lib/Drupal/Core/Extension/InfoParserDynamic.php
index 94d6e3c6d6..64db0a8ed9 100644
--- a/core/lib/Drupal/Core/Extension/InfoParserDynamic.php
+++ b/core/lib/Drupal/Core/Extension/InfoParserDynamic.php
@@ -131,7 +131,6 @@ public function parse($filename) {
         if (!empty($non_machine_name_dependencies)) {
           throw new InfoParserException("Theme module dependencies must be machine names (only underscores, digits, and lowercase letters). At least one dependency declared by {$parsed_info['name']} is not in machine name format: " . implode(',', $non_machine_name_dependencies));
         }
-
       }
     }
     return $parsed_info;
diff --git a/core/lib/Drupal/Core/Extension/ThemeExtensionList.php b/core/lib/Drupal/Core/Extension/ThemeExtensionList.php
index d6b13be8c0..ca42313926 100644
--- a/core/lib/Drupal/Core/Extension/ThemeExtensionList.php
+++ b/core/lib/Drupal/Core/Extension/ThemeExtensionList.php
@@ -142,14 +142,20 @@ protected function doList() {
     $this->fillInSubThemeData($themes, $sub_themes);
 
     foreach ($themes as $key => $theme) {
-      // After $theme is processed by buildModuleDependencies(), there is a
+      // After $theme is processed by buildModuleDependencies(), there can be a
       // `$theme->requires` array containing both module and base theme
       // dependencies. The module dependencies are copied to their own property
       // so they are available to operations specific to module dependencies.
-      if (!isset($theme->requires)) {
+      if (isset($theme->requires)) {
+        $theme->module_dependencies = isset($theme->requires) ? array_diff_key($theme->requires, $themes) : [];
+      }
+      else {
+        // Even if no requirements are specified, the theme installation process
+        // expects the presence of the `requires` and `module_dependencies`
+        // properties, so they should be initialized here as empty arrays.
         $theme->requires = [];
+        $theme->module_dependencies = [];
       }
-      $themes[$key]->module_dependencies = isset($theme->requires) ? array_diff_key($theme->requires, $themes) : [];
     }
     return $themes;
   }
diff --git a/core/tests/Drupal/Tests/Core/Extension/InfoParserUnitTest.php b/core/tests/Drupal/Tests/Core/Extension/InfoParserUnitTest.php
index 2e8c9ce1ca..c0ada9e028 100644
--- a/core/tests/Drupal/Tests/Core/Extension/InfoParserUnitTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/InfoParserUnitTest.php
@@ -534,4 +534,94 @@ public function testUnparsableCoreVersionRequirement() {
     $this->infoParser->parse(vfsStream::url('modules/fixtures/unparsable_core_version_requirement.info.txt'));
   }
 
+  /**
+   * Tests theme dependencies not in machine name format.
+   *
+   * @param string $theme_yml
+   *   YML of a theme depending on modules.
+   * @param string $exception_message
+   *   The expected exception message.
+   *
+   * @dataProvider providerTestUnparsableThemeModuleDependency
+   */
+  public function testUnparsableThemeModuleDependency($theme_yml, $exception_message) {
+    vfsStream::setup('modules');
+    vfsStream::create([
+      'fixtures' => [
+        'unparsable_module_dependency.info.txt' => $theme_yml,
+      ],
+    ]);
+    $this->expectException(InfoParserException::class);
+    $this->expectExceptionMessage($exception_message);
+    $this->infoParser->parse(vfsStream::url('modules/fixtures/unparsable_module_dependency.info.txt'));
+  }
+
+  /**
+   * Data provider for testUnparsableThemeModuleDependency().
+   */
+  public function providerTestUnparsableThemeModuleDependency() {
+    $has_version_constraints = <<<UNPARSABLE
+# info.yml for testing invalid core_version_requirement value.
+name: Test Theme Depending on Modules
+type: theme
+core: 8.x
+base theme: stark
+dependencies:
+  - test_module_required_by_theme
+  - module_with_some_version_metadata (>4.1)
+UNPARSABLE;
+
+    $has_project_prefix = <<<UNPARSABLE
+# info.yml for testing invalid core_version_requirement value.
+name: Test Theme Depending on Modules
+type: theme
+core: 8.x
+base theme: stark
+dependencies:
+  - test_module_required_by_theme
+  - drupal::module_with_a_project_prefix
+UNPARSABLE;
+
+    $has_a_space = <<<UNPARSABLE
+# info.yml for testing invalid core_version_requirement value.
+name: Test Theme Depending on Modules
+type: theme
+core: 8.x
+base theme: stark
+dependencies:
+  - test_module_required_by_theme
+  - apparently_i_have_a space
+UNPARSABLE;
+
+    $has_capital = <<<UNPARSABLE
+# info.yml for testing invalid core_version_requirement value.
+name: Test Theme Depending on Modules
+type: theme
+core: 8.x
+base theme: stark
+dependencies:
+  - test_module_required_by_theme
+  - capitalLetters
+UNPARSABLE;
+
+    return [
+      'has_version_constraints' => [
+        $has_version_constraints,
+        'Theme module dependencies must be machine names (only underscores, digits, and lowercase letters). At least one dependency declared by Test Theme Depending on Modules is not in machine name format: module_with_some_version_metadata (>4.1)',
+      ],
+      'has_project_prefix' => [
+        $has_project_prefix,
+        'Theme module dependencies must be machine names (only underscores, digits, and lowercase letters). At least one dependency declared by Test Theme Depending on Modules is not in machine name format: drupal::module_with_a_project_prefix',
+      ],
+      'has_a_space' => [
+        $has_a_space,
+        'Theme module dependencies must be machine names (only underscores, digits, and lowercase letters). At least one dependency declared by Test Theme Depending on Modules is not in machine name format: apparently_i_have_a space',
+      ],
+      'has_capital' => [
+        $has_capital,
+        'Theme module dependencies must be machine names (only underscores, digits, and lowercase letters). At least one dependency declared by Test Theme Depending on Modules is not in machine name format: capitalLetters',
+      ],
+    ];
+  }
+
 }