diff --git a/core/src/main/java/hudson/Launcher.java b/core/src/main/java/hudson/Launcher.java
index 382e850bbd43d26f8ed0f4e762a0b3e24e4ee568..bb5748827d63a0e42ec16bda8e3c5eeb1f3042bd 100644
--- a/core/src/main/java/hudson/Launcher.java
+++ b/core/src/main/java/hudson/Launcher.java
@@ -1087,9 +1087,10 @@ public abstract class Launcher {
      */
     private static EnvVars inherit(Map<String,String> overrides) {
         EnvVars m = new EnvVars(EnvVars.masterEnvVars);
-        // first add all values and then eventually expand them as values can refer other newly added values (see JENKINS-19488)
-        for (Map.Entry<String,String> o : overrides.entrySet()) 
-            m.override(o.getKey(),o.getValue());
+        // first add all new values and then eventually expand them as values can refer other newly added values (see JENKINS-19488)
+        for (Map.Entry<String,String> o : overrides.entrySet())
+            if(m.get(o.getKey()) == null)
+                m.put(o.getKey(), o.getValue());
         for (Map.Entry<String,String> o : overrides.entrySet()) 
             m.override(o.getKey(),m.expand(o.getValue()));
         return m;
diff --git a/test/src/test/java/hudson/LauncherTest.java b/test/src/test/java/hudson/LauncherTest.java
index c98f7e20bae56d59fbc451d80a2ca265dc2d1a5b..f20227d8a60c265df5898b69424aebe39e3fb063 100644
--- a/test/src/test/java/hudson/LauncherTest.java
+++ b/test/src/test/java/hudson/LauncherTest.java
@@ -25,19 +25,22 @@ package hudson;
 
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.containsString;
-
-import java.io.IOException;
-
 import hudson.model.FreeStyleBuild;
 import hudson.model.FreeStyleProject;
 import hudson.model.ParametersDefinitionProperty;
+import hudson.model.Slave;
 import hudson.model.StringParameterDefinition;
-import hudson.tasks.CommandInterpreter;
 import hudson.tasks.BatchFile;
+import hudson.tasks.CommandInterpreter;
 import hudson.tasks.Shell;
 
+import java.io.IOException;
+import java.util.HashMap;
+import java.util.Map;
+
 import org.junit.Rule;
 import org.junit.Test;
+import org.jvnet.hudson.test.Bug;
 import org.jvnet.hudson.test.JenkinsRule;
 
 public class LauncherTest {
@@ -45,6 +48,7 @@ public class LauncherTest {
     @Rule
     public JenkinsRule rule = new JenkinsRule();
 
+    @Bug(19488)
     @Test
     public void correctlyExpandEnvVars() throws Exception {
         FreeStyleProject project = rule.createFreeStyleProject();
@@ -63,6 +67,27 @@ public class LauncherTest {
 
         assertThat(log(build), containsString("aaa aaaccc ccc"));
     }
+    
+    @Bug(19926)
+    @Test
+    public void overwriteSystemEnvVars() throws Exception {
+        Map<String, String> env = new HashMap<String,String>();
+        env.put("jenkins_19926", "original value");
+        Slave slave = rule.createSlave(new EnvVars(env));
+        
+        FreeStyleProject project = rule.createFreeStyleProject();
+        project.addProperty(new ParametersDefinitionProperty(new StringParameterDefinition("jenkins_19926", "${jenkins_19926} and new value")));
+        final CommandInterpreter script = Functions.isWindows()
+                ? new BatchFile("echo %jenkins_19926")
+                : new Shell("echo ${jenkins_19926}")
+        ;
+        project.getBuildersList().add(script);
+        project.setAssignedNode(slave.getComputer().getNode());
+
+        FreeStyleBuild build = project.scheduleBuild2(0).get();
+
+        assertThat(log(build), containsString("original value and new value"));
+    }
 
     @SuppressWarnings("deprecation")
     private String log(FreeStyleBuild build) throws IOException {