diff --git a/src/com/cloudogu/ces/cesbuildlib/Docker.groovy b/src/com/cloudogu/ces/cesbuildlib/Docker.groovy
index 9937d91..aeacce2 100644
--- a/src/com/cloudogu/ces/cesbuildlib/Docker.groovy
+++ b/src/com/cloudogu/ces/cesbuildlib/Docker.groovy
@@ -401,7 +401,7 @@ class Docker implements Serializable {
             def userName = sh.returnStdOut('whoami')
             String jenkinsUserFromEtcPasswd = sh.returnStdOut "cat /etc/passwd | grep $userName"
 
-            if (jenkinsUserFromEtcPasswd.isEmpty()) {
+            if (jenkinsUserFromEtcPasswd == null || jenkinsUserFromEtcPasswd.isEmpty()) {
                 script.error 'Unable to parse user jenkins from /etc/passwd.'
             }
             return jenkinsUserFromEtcPasswd
diff --git a/src/com/cloudogu/ces/cesbuildlib/Sh.groovy b/src/com/cloudogu/ces/cesbuildlib/Sh.groovy
index d9521c3..14f2e98 100644
--- a/src/com/cloudogu/ces/cesbuildlib/Sh.groovy
+++ b/src/com/cloudogu/ces/cesbuildlib/Sh.groovy
@@ -18,6 +18,6 @@ class Sh implements Serializable {
                 // Trim to remove trailing line breaks, which result in unwanted behavior in Jenkinsfiles:
                 // E.g. when using output in other sh() calls leading to executing the sh command after the line breaks,
                 // possibly discarding additional arguments
-                .trim()
+                ?.trim()
     }
 }