diff --git a/src/BEMEWS/_ext/mstl/math2/number.h b/src/BEMEWS/_ext/mstl/math2/number.h
index 0392dc8..00dc467 100644
--- a/src/BEMEWS/_ext/mstl/math2/number.h
+++ b/src/BEMEWS/_ext/mstl/math2/number.h
@@ -15,6 +15,10 @@
 // M_PI, M_PI_2, M_PI_4, M_1_PI, M_2_PI, M_2_SQRTPI
 // M_SQRT2, M_SQRT1_2
 
+#if !defined(M_PI)
+#define M_PI 3.141592653589793238462643383279502884197169399375105820974944
+#endif
+
 #define M_SQRTPI 1.772453850905516027298167      // sqrt of pi
 #define M_1_SQRTPI 0.564189583547756286948       // reciprocal of sqrt pi 
 #define M_2PI 6.283185307179586476925286766559   // 2 pi