diff --git a/src/Console/Command/BuildCommand.php b/src/Console/Command/BuildCommand.php
index 5c511ce..0748461 100644
--- a/src/Console/Command/BuildCommand.php
+++ b/src/Console/Command/BuildCommand.php
@@ -32,6 +32,11 @@ final class BuildCommand extends AbstractCommand
             workingDir: $workingDir,
         ))->getLanguage();
 
+        assert(
+            assertion: ProjectLanguage::isValid($language),
+            description: sprintf('%s is not a supported language.', $language),
+        );
+
         // Attempt to prepopulate some of the options, such as the project type
         // based on its dependencies.
         // TODO: move this logic to a service so it can be tested.
@@ -60,6 +65,11 @@ final class BuildCommand extends AbstractCommand
         // with the option value if there is one.
         $projectType = $input->getOption('type') ?? $projectType;
 
+        assert(
+            assertion: ProjectType::isValid($projectType),
+            description: sprintf('%s is not a supported project type.', $projectType),
+        );
+
         $isDockerCompose = $this->filesystem->exists($workingDir . '/docker-compose.yaml');
 
         switch ($language) {
diff --git a/src/Console/Command/InstallCommand.php b/src/Console/Command/InstallCommand.php
index eddd2a3..7645e32 100644
--- a/src/Console/Command/InstallCommand.php
+++ b/src/Console/Command/InstallCommand.php
@@ -32,6 +32,11 @@ final class InstallCommand extends AbstractCommand
             ))->getLanguage();
         }
 
+        assert(
+            assertion: ProjectLanguage::isValid($language),
+            description: sprintf('%s is not a supported language.', $language),
+        );
+
         // TODO: Composer in Docker Compose?
         $process = Process::create(
             args: explode(separator: ' ', string: strval($args)),
diff --git a/src/Console/Command/PackageInstallCommand.php b/src/Console/Command/PackageInstallCommand.php
index 7677f6b..860f534 100644
--- a/src/Console/Command/PackageInstallCommand.php
+++ b/src/Console/Command/PackageInstallCommand.php
@@ -39,6 +39,11 @@ final class PackageInstallCommand extends AbstractCommand
             workingDir: $workingDir,
         ))->getLanguage();
 
+        assert(
+            assertion: ProjectLanguage::isValid($language),
+            description: sprintf('%s is not a supported language.', $language),
+        );
+
         switch ($language) {
             case ProjectLanguage::PHP->value:
                 $process = Process::create(
diff --git a/src/Console/Command/RunCommand.php b/src/Console/Command/RunCommand.php
index 968b3c6..882074c 100644
--- a/src/Console/Command/RunCommand.php
+++ b/src/Console/Command/RunCommand.php
@@ -32,6 +32,11 @@ final class RunCommand extends AbstractCommand
             workingDir: $workingDir,
         ))->getLanguage();
 
+        assert(
+            assertion: ProjectLanguage::isValid($language),
+            description: sprintf('%s is not a supported language.', $language),
+        );
+
         // Attempt to prepopulate some of the options, such as the project type
         // based on its dependencies.
         // TODO: move this logic to a service so it can be tested.
@@ -60,6 +65,11 @@ final class RunCommand extends AbstractCommand
         // with the option value if there is one.
         $projectType = $input->getOption('type') ?? $projectType;
 
+        assert(
+            assertion: ProjectType::isValid($projectType),
+            description: sprintf('%s is not a supported project type.', $projectType),
+        );
+
         $filesystem = new Filesystem();
         $isDockerCompose = $filesystem->exists($workingDir . '/docker-compose.yaml');
 
diff --git a/src/Enum/ProjectLanguage.php b/src/Enum/ProjectLanguage.php
index f0df04c..39bd65d 100644
--- a/src/Enum/ProjectLanguage.php
+++ b/src/Enum/ProjectLanguage.php
@@ -8,4 +8,16 @@ enum ProjectLanguage: string
 {
     case JavaScript = 'javascript';
     case PHP = 'php';
+
+    /**
+     * @param non-empty-string $language
+     */
+    public static function isValid(string $language): bool
+    {
+        return in_array(
+            haystack: array_column(self::cases(), 'value'),
+            needle: $language,
+            strict: true,
+        );
+    }
 }
diff --git a/src/Enum/ProjectType.php b/src/Enum/ProjectType.php
index f5f77b2..ae593b7 100644
--- a/src/Enum/ProjectType.php
+++ b/src/Enum/ProjectType.php
@@ -13,4 +13,16 @@ enum ProjectType: string
     case Drupal = 'drupal';
     case Sculpin = 'sculpin';
     case Symfony = 'symfony';
+
+    /**
+     * @param non-empty-string $projectType
+     */
+    public static function isValid(string $projectType): bool
+    {
+        return in_array(
+            haystack: array_column(self::cases(), 'value'),
+            needle: $projectType,
+            strict: true,
+        );
+    }
 }