diff --git a/src/build/generators.jam b/src/build/generators.jam
index e87d2ebb6..b2cab15ba 100644
--- a/src/build/generators.jam
+++ b/src/build/generators.jam
@@ -121,7 +121,15 @@ rule generator (
         self.target-types += $(m[1]) ;
         self.name-pre-post.$(m[1]) = $(m[3]) $(m[4]) ;
     }
+
+    # Note that 'transform' here, is the same as 'for_each'.
+    sequence.transform type.validate : $(self.source-types) ;
+    if $(self.target-types) != *
+    {        
+        sequence.transform type.validate : $(self.target-types) ;
+    }
     
+        
     self.optional-properties
       = [ feature.expand <base-target-type>$(self.target-types) ]
           ;
@@ -478,7 +486,7 @@ rule generator (
 
 class generator ;
 
-rule composing-generator ( id : source-types + : target-types + :
+rule composing-generator ( id : source-types * : target-types + :
                            requirements * )
 {
     generator.__init__ $(id) : $(source-types) : $(target-types) :
diff --git a/src/build/type.jam b/src/build/type.jam
index 31dd4ef60..5eaabaa44 100644
--- a/src/build/type.jam
+++ b/src/build/type.jam
@@ -109,6 +109,16 @@ rule registered ( type )
     }
 }
 
+# Issues an error if 'type' is unknown.
+rule validate ( type )
+{
+    if ! $(type) in $(.types)
+    {
+        errors.error "Unknown target type $(type)" ;
+    }    
+}
+
+
 # Sets a scanner class that will be used for this 'type'.
 rule set-scanner ( type : scanner )
 {
diff --git a/src/tools/builtin.jam b/src/tools/builtin.jam
index 7f0d2a424..6db1996a4 100644
--- a/src/tools/builtin.jam
+++ b/src/tools/builtin.jam
@@ -291,7 +291,7 @@ type.register C : c ;
 # "shared" feature.
 rule lib-generator ( ) 
 {
-    composing-generator.__init__ lib-generator : unknown-source-type : LIB : <main-target-type>LIB ;
+    composing-generator.__init__ lib-generator : : LIB : <main-target-type>LIB ;
     
     rule run ( project name ? : properties * : sources * )
     {