src/Normalizer/Value/FieldNormalizerValue.php | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/Normalizer/Value/FieldNormalizerValue.php b/src/Normalizer/Value/FieldNormalizerValue.php
index a0434fa..e43e4f2 100644
--- a/src/Normalizer/Value/FieldNormalizerValue.php
+++ b/src/Normalizer/Value/FieldNormalizerValue.php
@@ -70,6 +70,7 @@ class FieldNormalizerValue implements FieldNormalizerValueInterface {
     }
 
     if ($this->cardinality == 1) {
+      assert(count($this->values) === 1);
       return $this->values[0] instanceof FieldItemNormalizerValue
         ? $this->values[0]->rasterizeValue() : NULL;
     }