diff --git a/pjlib/build/pjlib.dsp b/pjlib/build/pjlib.dsp
index e5b8011..39d800c 100644
--- a/pjlib/build/pjlib.dsp
+++ b/pjlib/build/pjlib.dsp
@@ -553,6 +553,10 @@
 # End Source File
 # Begin Source File
 
+SOURCE=..\include\pj\math.h
+# End Source File
+# Begin Source File
+
 SOURCE=..\include\pj\os.h
 # End Source File
 # Begin Source File