diff --git a/vars/buildStep.groovy b/vars/buildStep.groovy
index e5de1a4..a24c099 100644
--- a/vars/buildStep.groovy
+++ b/vars/buildStep.groovy
@@ -10,8 +10,8 @@ Map defaults = [
 
 def call(Map config = [:]) {
     assert config.artifacts : 'must specify artifacts!'
-    String runConfig = defaults + config
-    if ( fileExists("${runConfig.buildTest}")) {
+    Map runConfig = defaults + config
+    if ( fileExists(runConfig.buildTest)) {
         sh """
             ${runConfig.buildCmd}
         """