diff --git a/src/Logger/FilteredLogLevelDecorator.php b/src/Logger/FilteredLogLevelDecorator.php
index 5af4b62e..465e5fdc 100644
--- a/src/Logger/FilteredLogLevelDecorator.php
+++ b/src/Logger/FilteredLogLevelDecorator.php
@@ -8,6 +8,9 @@
 use Psr\Log\LoggerTrait;
 use Psr\Log\LogLevel;
 use Webmozart\Assert\Assert;
+use function array_keys;
+use function implode;
+use function sprintf;
 use function strtolower;
 
 /**
@@ -41,7 +44,15 @@ final class FilteredLogLevelDecorator implements LoggerInterface
      */
     public function __construct(LoggerInterface $realLogger, string $minimumLogLevel)
     {
-        Assert::keyExists(self::LOG_LEVEL_ORDER, strtolower($minimumLogLevel));
+        Assert::keyExists(
+            self::LOG_LEVEL_ORDER,
+            strtolower($minimumLogLevel),
+            sprintf(
+                'Log level %s was not a valid PSR-3 compatible log level. Should be one of: %s',
+                $minimumLogLevel,
+                implode(', ', array_keys(self::LOG_LEVEL_ORDER))
+            )
+        );
 
         $this->minimumLogLevel = self::LOG_LEVEL_ORDER[strtolower($minimumLogLevel)];
         $this->realLogger      = $realLogger;
diff --git a/tests/Unit/Logger/FilteredLogLevelDecoratorTest.php b/tests/Unit/Logger/FilteredLogLevelDecoratorTest.php
index 5a8888b1..8be37fcf 100644
--- a/tests/Unit/Logger/FilteredLogLevelDecoratorTest.php
+++ b/tests/Unit/Logger/FilteredLogLevelDecoratorTest.php
@@ -28,6 +28,16 @@ public function setUp() : void
         $this->decoratedLogger = $this->createMock(LoggerInterface::class);
     }
 
+    public function testInvalidLogLevelStringGivesClearErrorMessage() : void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage(
+            'Log level foo was not a valid PSR-3 compatible log level. '
+            . 'Should be one of: debug, info, notice, warning, error, critical, alert, emergency'
+        );
+        new FilteredLogLevelDecorator($this->decoratedLogger, 'foo');
+    }
+
     public function testLogMessagesHaveScoutTagPrepended() : void
     {
         $decorator = new FilteredLogLevelDecorator($this->decoratedLogger, LogLevel::DEBUG);