diff --git a/WHATSNEW b/WHATSNEW
index 7772cb580..93abe4238 100644
--- a/WHATSNEW
+++ b/WHATSNEW
@@ -10,6 +10,10 @@ Fixed bugs:
  * Encoding of unicode escape sequences by the property file task
    Bugzilla Report 50515.
 
+ * The code that implicitly sets the -source switch if only -target is
+   in <javac> was broken for Java 5 and 6.
+   Bugzilla Report 50578.
+
 Other changes:
 --------------
 
diff --git a/src/main/org/apache/tools/ant/taskdefs/compilers/DefaultCompilerAdapter.java b/src/main/org/apache/tools/ant/taskdefs/compilers/DefaultCompilerAdapter.java
index b678984c5..e129f02af 100644
--- a/src/main/org/apache/tools/ant/taskdefs/compilers/DefaultCompilerAdapter.java
+++ b/src/main/org/apache/tools/ant/taskdefs/compilers/DefaultCompilerAdapter.java
@@ -361,7 +361,7 @@ public abstract class DefaultCompilerAdapter
                 setImplicitSourceSwitch((assumeJava15() || assumeJava16())
                                         ? "1.5 in JDK 1.5 and 1.6"
                                         : "1.7 in JDK 1.7",
-                                        cmd, s, t);
+                                        cmd, t, s);
             } else if (assumeJava17() && (t.equals("1.5") || t.equals("1.6"))) {
                 setImplicitSourceSwitch("1.7 in JDK 1.7", cmd, t, t);
             }