diff --git a/src/libFLAC/window.c b/src/libFLAC/window.c
index 33c319f7..a13a21f9 100644
--- a/src/libFLAC/window.c
+++ b/src/libFLAC/window.c
@@ -36,6 +36,11 @@
 
 #ifndef FLAC__INTEGER_ONLY_LIBRARY
 
+#ifndef M_PI
+/* math.h in VC++ doesn't seem to have this (how Microsoft is that?) */
+#define M_PI 3.14159265358979323846
+#endif
+
 
 void FLAC__window_bartlett(FLAC__real *window, const FLAC__int32 L)
 {