diff --git a/src/Executor/ReferenceExecutor.php b/src/Executor/ReferenceExecutor.php
index 837628a..5944a6e 100644
--- a/src/Executor/ReferenceExecutor.php
+++ b/src/Executor/ReferenceExecutor.php
@@ -153,7 +153,7 @@ class ReferenceExecutor implements ExecutorImplementation
                     break;
             }
         }
-        if (! $operation) {
+        if ($operation === null) {
             if ($operationName) {
                 $errors[] = new Error(sprintf('Unknown operation named "%s".', $operationName));
             } else {
@@ -165,7 +165,7 @@ class ReferenceExecutor implements ExecutorImplementation
             );
         }
         $variableValues = null;
-        if ($operation) {
+        if ($operation !== null) {
             [$coercionErrors, $coercedVariableValues] = Values::getVariableValues(
                 $schema,
                 $operation->variableDefinitions ?: [],